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T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

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TK77FR - Custom Fit Tekonsha Trailer Hitch Wiring
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Quickly and easily install a 4-way trailer connector on your vehicle with this custom wiring harness. T-One connector plugs directly into existing wiring. Powered ModuLite reduces strain on wiring system. Call 800-298-1624 to order Tekonsha custom fit vehicle wiring part number TK77FR or order online at etrailer.com. Free expert support on all Tekonsha products. Guaranteed Lowest Price and Fastest Shipping for T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector. Custom Fit Vehicle Wiring reviews from real customers.
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  • Videos (2)
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Tekonsha Custom Fit Vehicle Wiring

  • Custom Fit
  • Trailer Hitch Wiring
  • 4 Flat
  • Converter
  • Tekonsha

Quickly and easily install a 4-way trailer connector on your vehicle with this custom wiring harness. T-One connector plugs directly into existing wiring. Powered ModuLite reduces strain on wiring system.


  • Provides a 4-way flat trailer connector at the back of your vehicle to power your trailer's signal lights
    • Compatible with 5-pole, 6-pole and 7-pole adapters (sold separately)
  • Connects quickly and easily
    • Locate connection points in rear cargo area
    • Plug in T-connector
    • Run hot lead to car battery
  • Includes ModuLite power module that connects directly to battery to avoid overloading vehicle's electrical system
  • Constructed of 16-gauge bonded wire
  • Comes with dust cap for 4-way flat connector


  • Maximum amperage:
    • Stop/turn lights: 5 amps per circuit
    • Tail lights: 7.5 amps per circuit
  • 1-Year limited warranty


This custom-fit T-One connector plugs into your vehicle's wiring harness, which is located in the rear cargo area. You will have to remove the plastic threshold cover, side panels, cargo trays, and interior trim panels to access this harness.

Once you've plugged in the T-connector, run the hot lead up to your car battery. Connect the lead to the included fuse holder, and then connect the fuse holder to the positive terminal on your battery and insert the provided fuse.

Once installation is complete, the 4-way flat connector will be stowed in your rear cargo area.

It is recommended that you use a small amount of grease on all electrical connections - the plugs on your automobile and the 4-pole connector itself - to help prevent corrosion.

ModuLite Power Module

A ModuLite power module is built into this T-One connector. With a ModuLite, your trailer's lights will be powered without overloading your vehicle's electrical system. Because the module draws power directly from your car's battery and bypasses the electrical routes that are used by a basic wiring harness, there is virtually no draw on your tail light circuits.

118823 Tekonsha T-1 T-Connector - 4 Wire - Modu-Lite Converter Included

Item # TK77FR

Installation Details TK77FR Installation instructions

This Product Fits The Following Vehicles

California residents: click here

Video of T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

Videos are provided as a guide only. Refer to manufacturer installation instructions and specs for complete information.

Video Transcript for Tekonsha T-One Vehicle Wiring Harness Installation - 2019 BMW X3

Hi there, BMW owners. Today, on your 2019 BMW X3, we're gonna be taking a look at and showing you how to install Tekonsha's T-One vehicle wiring harness. And this is what our harness looks like when it's installed. That's right, you can't see it anywhere here at the back because it's hidden inside of our vehicle. This is gonna allow us to get a more factory look here at the back. And it's also gonna provide better protection for our connector, 'cause it's not gonna be exposed to moisture and all of the dirt, debris and humidity that's outside.

So, a lot of connectors that stay outside, if you don't religiously grease them with dielectric grease, and even if you do, they get susceptible to corrosion and they end up requiring repairs and replacement sooner than the one that's just stored in the back of the vehicle here. So, you can find it underneath your compartment here. You just lift it up, and that's where you'll find your wiring, stored. We can simply just take this out. Your dust boot, located on your wiring works really well to secure all your wiring together here.

So that way, it's a smaller bundle when it's inside the vehicle. It helps keep things nice and neat. We're just gonna undo that now. And our wiring will just drape right out the back here. We can now shut our liftgate, and it's just gonna pinch in between our weatherstripping.

It's not gonna cause any damage to the wiring. It's gonna hold it right in place for us right there. The only thing you wanna make sure, that when you're closing your liftgate, is you avoid the striker here at the middle. This can damage your wiring. But the rest of these spots are gonna be fine, anywhere in these locations here.

So, we're just gonna go ahead and shut that now. And you can see there, it holds it nice and secure there at the back. We've got plenty of lengths here to be able to plug into our trailer. And if you're doing maybe a little bit of moving around, maybe you're moving a trailer here and there, getting a little bit of work done, dropping a couple trailers off at one place or another and using your wiring all day long, you can use that dust boot to secure it to the safety chain loop on your hitch. That way, it's ready for you when you're out adventuring and whatever you're doing that day that you need to be using this constantly on and off, on and off, you can easily access it right there real fast. When we do plug it into our trailer, we've got four connections here. It's gonna provide us with all of our necessary lighting signals, which includes our left turn signal, right turn signal, tail lamps, and brake lamps. That's gonna keep us DOT compliant in all states. And it'll also ensure that the drivers behind us will know your intentions when going down the road. This harness is a module-protected harness, so there's a module that comes included with this harness and that module will monitor the inputs from your vehicle. It's a custom-fit harness, so it has custom-fit connectors that plugged directly into the taillight assemblies behind your vehicle. And that's how the module will then monitor those signals. It just intercept, so many can see that when the brakes or turn signals are coming on on the car, that module will then reproduce those signals and send them out our four-pole connector here. What's nice about the module reproducing those signals is that if you have any faults or issues with your trailer, let's say, you had a short on there, typically, if you we're hooked directly to the lights on your vehicle here, it would open the fuse and then the lights in your vehicle wouldn't work. But since we have a module in between those circuits, the module will either shut down that circuit if it detects a short, or in the event it's unable to shut it down in time, the module has its own dedicated power circuit with its own fuse, it would just open up that fuse and that would just mean your lighting on your trailer would cease to function, but nothing on your vehicle would be affected, so that's really nice. That really protects our vehicle from any issues that could occur with faulty wiring on our trailer. When your trailer sits outside, you can see, it has a lot of corrosion, a lot of things going on, so that's usually where you end up finding issues is on the trailer side. And sometimes, that can trickle back to your vehicle if you don't have proper protection. So, a module like this, with it custom-fit, it's gonna make your life really easy 'cause the install is quite simple. Actually, one of the most simple ones that I've done compared to some other vehicles that you would expect to be easier. BMWs are usually fairly involved, but this one's really easy. So, why don't you follow along with us and we'll show you how to get it installed. It should be able to be done in about, I would say about a half an hour. You could probably get this done at home. And there's really no special tools or anything required. You are gonna need your little socket set and some screwdrivers, but nothing crazy. We'll begin our installation here at the back of the vehicle with our liftgate open. We need to gain access to the connectors located behind our passenger and driver's side taillight assemblies. To do this, you'll have these compartments right here. There's a handle right here on top. You simply just pull out and then up to remove this. We're just gonna set that in the seat right up there. We're gonna remove the other side as well, and set that out of our way also. We'll now get the connector disconnected off of our taillights. On the driver's side here, it can be found in this slit here. This wiring here is actually our taillight wiring, so just kinda peel this back out of the way. And there you can see your connector in there. Now, it can be a bit difficult to see, but there is a small release tab. And we're gonna take our screwdriver here. We're gonna press in on that release tab there, and then just take our screwdriver and just pry it right out of there. We'll then go over to the passenger's side and do the same thing. There are two clips we need to remove over there though, because this fabric covers this completely up. So, now we're here on the passenger's side where we're talking about those two clips. Your screwdriver will go in the little slot. There's a slot located on each side of the clip. Just twist your screwdriver once you've put it in the little slot, and then you can pull the pin out. We'll do the same thing with the second clip further up here. We'll get that one out of the way as well. And now, we can peel our fabric back here to reveal the connector going to the passenger's side taillight assembly. This is gonna disconnect the exact same way. We can now take our custom-fit wiring harness. You'll have your module here, and coming out of that, you're gonna have several wires. We wanna focus on the harness that has the yellow and the brown wire. This is our driver's side connector right here. You can see, we've got factory connector ends on it, so this is gonna plug directly into our vehicle. Nice custom-fit harness, so we don't have to modify the vehicle whatsoever. We'll just take our connector here. We're going to just plug that in there. And then the other end here is gonna plug right into our taillight assembly. So, we're just gonna peel this fabric out of the way. Line our connector up. It is a tight fit, so it is gonna be a little bit difficult, but you should be able to get it in there and it'll click in place. Give her just a little light tug to make sure it doesn't just slide right out of there. We're not pulling real hard. We don't wanna pull our pins out. Just kinda making sure that it's not just gonna fall out of there. So, now we've got those connected, we're gonna be hooking up the passenger's side next. That's the green wire located here. You might have a rubber band down there, just get that removed. We're gonna take the module and we're just gonna drop everything right into that little bucket there. We'll then take our cover here. We're gonna lift up on this. And there's this hooks, you can use that to your advantage to hold the panel up out of your way. We need to route this green wire across to the passenger's side. So, we're gonna lift up this portion of the carpet right here. Take your screwdriver and just twist these counter-clockwise, they should pop up, just like that. And there's one on the other side, we'll pop that one up. And that's really as far as we need to get. We can access everything we need to right there. So, now that we have access there, we need to figure out how we're gonna get our wiring routed over there. So, we're just gonna take our green wire now. We're gonna route this underneath. We can stay underneath our carpet. And then kinda wrap it around, trying to hide it as good as possible underneath any of your panels here. Kinda lift up and just tuck it under there and run it across. Once we get to the other side, we need to feed it through, so we can access our connectors over here. You can see the factory wiring right there. We just followed that factory wiring to kinda went back and it came out right there behind the compartment. 'Cause then we can just kinda route it around this compartment. Just tuck it right around the outside of it. Get back here to our taillight assemblies. Plug into the factory connector. And then plug our other connector into our taillight assembly. Our wires can just tuck right down, like that. And we can even put a couple of zip ties on there if we wanted to, to keep it back. But the fabric here should keep everything pretty well in place. Next, we took our four-pole wiring and we routed this underneath as well. But we stayed above the carpet panel here this time, because this is where it's going to store when you're not using it. It actually stores in the storage tray here in the back. But when you wanna use it, you'll just drape it out the back. So, we wanna make sure we've got that in that tray there. And we have two circuits left on here. We've got our power circuit, which we'll need to run to the battery, and our ground circuit. So, we're gonna look for a ground wire over in this location here. And really, any of these bolts are gonna be great candidates. So, there's a good chance that we're gonna be mounting our module probably right here. So, it's a good idea just to kinda just figure out where you're probably gonna place your module, so you can see, hey, what lengths of wire do I got here Where's a good place for me to actually make my connection And this looks like a really good connection for our ground right there. So, we'll grab a 10-millimeter socket to remove that bolt. We'll then take our ring terminal. We're gonna slide our bolt through the ring terminal. And then we're just going to reinstall the bolt. Now we've got our ground connection made. The last connection we need to make is to our battery. And our batteries actually located underneath the carpet that we had just routed our green wire across with. Here's our battery right here. So, that actually makes things pretty simple. You're gonna receive a lot of extra black wire in your kit. 'Cause in most cases, on a lot of vehicles, the batteries located in the engine compartment up at the front of the vehicle, so that's why you get so much. So, we're gonna go ahead and connect this now to the black wire off of our module and route it towards the battery here. You're gonna expect to have quite a bit of extra wire. Next, we're gonna route our black wire and we need to get that behind the compartment here. I did actually just remove the green wire too. Instead of going over here, I found that I could reach my hand under here and pull it out of the compartment here. It's just a little bit cleaner. And that's the way that I'm gonna be routing my black wire as well. So, if I'm poking there, we can feed the wire over and we should be able to pull it up through the compartment there. Additionally, you could also go the other direction. We could go down. Whichever one seems to be easier for you. That's the way I'd recommend. There it is. So, there we go. And we just need to make it to the battery. So, that's really all the length of wire that we really need. 'Cause we're also gonna be installing a fuse harness between our battery and our circuit, so we have proper circuit protection. So, we've got plenty pulled through to the battery now. So, then this side here is gonna connect to our module. So, we'll go ahead and grab our strippers and our crimpers to make our connections. So, we're gonna cut our wire about here. This will give us a little bit of excess on each side. I always like having a little bit of excess for future repairs or maybe future accessory installs. Sometimes, it just makes your life a little bit easier. Now, that we've got that cut, we can take the black wire and we're gonna strip it back. And we're gonna connect it to the black wire coming off of our module here. We can go ahead and use the butt connectors that came provided with our kit. Since these are regular butt connectors, I wouldn't recommend using them outside the vehicle, but since all of our components are inside the vehicle here, we don't really have to worry about rust and corrosion, things like that, about moisture, so we're fine to just use these. Slide our wire in one end, crimp it down. And then we'll take the black wire on the other end, slide it in, crimp it down. And then we can go ahead and get this module mounted up. Included with your kit, you're gonna get some double-sided adhesive. That's what we're gonna be using to mount our module. We know we wanna put it roughly in this location here, so I just put a little bit of alcohol on a rag. We're just gonna clean that surface to make sure we get good adhesion with our double-sided tape. Also, we're gonna go ahead and wipe off the backside of our module as well. We'll remove the adhesive backing from one side of our tape. We're gonna apply this to the backside of our module. We can then remove the other side of the adhesive tape. And we're just gonna peel back that carpeting a little bit. And then just to fix this right into place there. So, now we've got our black wire routed in. Our battery positive is right here, but the connection type that's right here where it crimps around the battery is not really a good spot to put a ring terminal. So, we're gonna continue on right past the battery, because we've got some very large battery positive connections that we can make right here. So, we're just gonna pop this up. And we've got few circuits right here. We're just gonna take off one of those nuts and we'll be attaching directly to that. This is our fuse harness that comes in our kit though. This needs to go in line, and before we connect our wiring directly to power. So, you'll just take your fuse harness. We're gonna cut it in half. And then we'll strip back each end of the fuse harness. One end is gonna receive a ring terminal and the other end is gonna receive a butt connector. And again, since we're inside the vehicle, we don't need to worry about corrosion or anything, so the included butt connectors are gonna be just fine. We'll slide that on one end, and we'll get this crimped on here. And then on the other end, we're gonna take that ring terminal, slide that into place, and get this crimped on. And then I like to hook the fuse harness to the battery first, 'cause then we can do another assessment to determine like, hey, is our wiring, we have way too much excess here. Do we need to trim a little bit more off It's a little bit easier if you've got that there. So, now we're gonna swap over to a 13-millimeter socket, and we're just gonna remove the nut. We'll take our fuse harness here. I'm just gonna place the wire off to the side there. And you can kinda maybe even slide it through that little slot. And that way, it'll stay out of the way when we go to reinstall the cover that goes on here. So, we'll just slide the nut into place. I've taken our nut, reinstalling it. We'll tighten that down. And then we can go ahead and put that plastic cover back into place. And it looks like we do need to make a small modification right here. We'll just take our snips. And we're just gonna cut out just a little bit of this orange piece, so we just snipped it right on the side. And then take our snips, and a lot of times, we can just fold this up and kinda twist it. It usually breaks off fairly clean there. And now we can fit it and see how it works. And that looks like that snapped right on there for us, so we're good to go. So, now we're just bringing over our black wire, checking our length. This really isn't too bad. We've got about six, seven inches of extra length. But that's fine, it'll sit in this area here. That's nothing too excessive. So, we're just gonna strip back now the black wire. And this is just gonna crimp to the other side of our fuse harness. And then we can crimp this down. And now, included in your kit, you should have received a 15 amp fuse. We are now ready to insert this into our fuse harness. And our system should be operational at this point. So, now is a really good time to just test your system out and make sure it's all working properly. I usually recommend doing that before you go reinstalling all your panels and stuff, in case you need to make any little adjustments. So, we're gonna go ahead and test out our wiring now. Your wiring is designed to drape over the back and pinch in between the weatherstripping when in use, so we're just gonna set it up like a normal use. We just wanna make sure that when we're closing it, we avoid the striker in the middle. That could potentially damage your wiring. With anywhere around that, you're fine to pinch it in place and it'll hold it nice. So, now I've gone ahead and plugged in a tester in the back. You could plug into your trailer, but if you have any faults on your trailer, it could give you false readings if there's something not working properly on your vehicle, so I highly recommend a tester like this. You can pick one up here at etrailer. And this way, you know for sure that your vehicle is working properly, so when you plug it into the trailer, if you have any issues, you know the faults on your trailer's side. So, we wanna make sure we've got all the necessary lighting signals. We've started the vehicle to make sure that we get that output. And you wanna check your left turn signal, right turn signal, tail lamps, and your brake lamps. With all the signals working properly, we're now ready to put our vehicle back together. We've also included zip ties, so you can clean up any of that wiring to make it nice and neat. And once we've got our panels reinstalled, we're ready to hook up our trailer and hit the road. And that completes our installation of Tekonsha's T-One vehicle wiring harness on our 2019 BMW X3..

Customer Reviews

T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector - TK77FR

Average Customer Rating:  5.0 out of 5 stars   (5 Customer Reviews)

Quickly and easily install a 4-way trailer connector on your vehicle with this custom wiring harness. T-One connector plugs directly into existing wiring. Powered ModuLite reduces strain on wiring system.

- TK77FR

Looks good, hopefully it is plug & Play

- TK77FR

Very easy

- TK77FR

- TK77FR

- TK77FR


Ask the Experts about this Tekonsha Custom Fit Vehicle Wiring

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Info for this part was:

Employee Lindsey S
Edited by:
Lindsey S
Employee Kathleen M
Installed by:
Kathleen M
Employee Chris R
Video Edited:
Chris R
Employee Wilson B
Updated by:
Wilson B
Employee Alexander C
Written by:
Alexander C
Employee David F
Test Fit:
David F
Employee Thomas T
Video by:
Thomas T

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