Trailer Wiring Harness Installation - 2015 Mercedes-Benz M-Class

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How to Install a Trailer Wiring Harness on a 2015 Mercedes-Benz M-Class

Today on our 2015 Mercedes M Class ML350 we're going to be installing Tekonsha's universal four pole trailer wiring kit, part number 119147KIT.This universal wiring kit is going to provide you with all the necessary lighting you need for your trailer. This includes your left turn signal, right turn signal, tail lamps, and brake lamps, so you'll be DOT compliant in all states. What's great about this kit compared to other kits that are specific to a particular year, make and model vehicle, this one being universal will work with just about any type of system out there, so you can order this with confidence that it will work with your system. This is also an ideal kit for any custom projects that you're working on that you're going to be hauling a trailer with because you know regardless of however you've got your lights rigged up in the back this will quick splice and connect right to it and give you those necessary lighting.The module on this kit has several connections for various outputs. You'll make those connections as necessary. The module will then supply all those signals to your four pole connector.

What's nice about this is that the module itself has its own separate fuse and power wire that's run, so if there's any faults on your trailer it will just open that fuse and it won't affect any of the lighting in your vehicle. On our Mercedes M Class the harness is going to live inside the vehicle. What's nice about this is that you won't have to worry about any corrosion or anything getting on it from being outside all the time. It can just tuck back and stay inside your side compartment.When you're ready to use it, you'll just drape it out over the back. You want to make sure to avoid the anchor point for your rear hatch, and you'll just close the back.

It will pinch down between your weather stripping. That won't cause any damage to the harness, but it will keep it in place. On our end here we'll have a dust cap so if we do leave it outside, we're not using it, but we're still driving around, we can just throw the cap on it and that will keep out any dirt and debris. Additionally, if you had it outside you can also use it to secure it to your hitch. That will keep it up from dragging on the road.We'll begin our installation at the back of the vehicle with our hatch open.

We're going to need to remove the threshold here at the back. You'll need a T30 torque socket to remove the bolt located back here. Now you can just pop it up. You'll want to start at one side and just work your way up going across. Once you get to the middle you may want to stop, go back to the other side, and get it started.

We can now set this aside.Next we'll need to remove the access panels that will allow us to get to our tail light wiring. There's going to be a cover on each side. You're just going to want to pull that back, remove it. We're going to do the same thing on the other side. Now we can access our wiring. Now on our driver's side we're going to disconnect the wiring connector that goes to our tail lamp. That's this one here. You'll want to push in on the release tab and then just pull the connector away to unplug it.We'll now take the quick splice that come with your kit and we're going to make our connections to our wiring that we just disconnected. The black and white wire will connect to the red wire from our new module's harness. We're just going to slide that quick splice on there. We're going to slide the red wire from our module through there as well. Then we'll take a pair of pliers or channel locks and we're just going to squeeze the metal tab down to connect those two wires together. Once you've done that you can just flip over the lock tab. We're going to repeat that same procedure with the yellow wire to our black wire. That's going to be our turn signal wire. Lastly, on our driver side we'll be using the gray and black wire from our harness and quick splicing onto that will be the brown wire from our new module. We can now plug our connector back into our tail light.Next we'll take our white wire coming off of our module with the ring terminal. We're going to take the self-tapping screw that comes in the kit, slide it through that ring terminal, use a quarter inch socket to tap that screw into the sheet metal right about in this location. Now we'll take the black wire that comes in our kit and we're going to route it where we had removed our threshold over to the cavity here. Since the black wire is a lot thicker and easier to work with when routing it through small passageways like this, we're going to go ahead and use this black wire to feed our green wire, which needs to go to the passenger side, to the other side. I'm just going to use some electrical tape, tape it to our black wire, and we'll pull that back through. We don't want to pull it all the way through. We want to leave our black wire here. We just want to use it to get our green wire to this point.Next we'll take our green wire that we had ran through our channel. We're going to strip it back. We're going to strip back the black wire off of our module. Then we'll crimp those together using the yellow butt connector that comes in your kit. Now we'll take the double sided tape that comes in our kit, we're going to peel off some of the adhesive backing. We're going to attach it to our module. We'll then peel back the adhesive backing on the opposite side and we're just going to stick it right onto the metal in this area back here. You'll just want to hold it there for about 20 seconds to make sure you've got good adhesion.We'll now take our green wire, we're going to run that across our threshold here. Then we'll poke it through the paneling into our cavity over here. If your green wire doesn't want to go through that cavity, you may need to use your black wire again because it is a thicker wire so it makes it a little easier to work with. We're going to do just like we did last time. We're just going to tape our green wire to our black wire and use that to pull it through. We can now separate the two. You can pull your black wire back out as we don't need our black wire in on this side.Now here on our passenger side we're going to disconnect the wiring connector for the passenger side tail light. We'll do that in the exact same way. You'll just press in on the release tab and then pull the wire out. We only have one connection to make on this side, and that's going to be to the violet and black wire. We're just going to push some of that sheathing back. Make sure you're holding it by the wires when you're pushing this back because you don't want to pull the wires out of the connector. You just need enough to isolate that violet and black wire. Slide our quick splice on it. Take our green wire and then just squeeze that down on our wires. I'll again close the locking tab and then plug our connector back in.We'll now take our black wire that we've run across here. We're going to route this up. We're just going to follow this carpeting right here and just tuck it behind that. We're going to run it up to the fuse box located underneath our passenger rear seat. After we've got the wire routed up along the side, we tucked it down behind the seat. We're going to use airline tubing to go through the little opening in the carpet here where our fuse compartment is here. We're just going to do like we do before. We're going to tape that wiring on there. This is a piece of airline tubing. It's a little bit thicker than our black wire, so it makes it a little easier to route it through difficult tight spaces. Now we'll just pull this black wire on through and we can keep that tucked up out of the way.Here in our fuse compartment press both of these tabs to open it up. We'll be connecting it to the stud right here. We can go ahead and just take our wire, we're going to cut off the excess. I do like to leave just a little bit just in case we need some for future repairs, so we can cut it at about that point. We'll then cut that excess black wire off. We can go ahead and strip back that black wire. Now we'll take the fuse harness that comes in the kit. We're going to cut it in half. Strip back each end of our harness. We'll connect a yellow ring terminal to one end and a yellow butt connector to the other end. The other end of our yellow butt connector we'll connect to the black wire that we ran up to our fuse box.Now we can take a 13 millimeter socket, we're going to remove the nut from the stud there in our fuse box. Now be careful this wire is live, so we do not want to ground it. You may want to disconnect your battery under the hood before doing this just to prevent any errors, any accidents. We're just going to slip that up. We're going to slide our ring terminal onto the stud. Then we're just going to reconnect our nut back onto the stud. I like to make sure that the ring terminal is oriented in a way to where the lid will shut, so we're just going to put it over here on the side like that and then tighten it back down. We can now close the lid on our fuse box and insert the 10 amp fuse that comes in your kit into the fuse harness. Close the lid and we can now test everything out.I like to test everything out before I put everything back together just in case you had a poor connection in any butt connector that you need to go back and fix up. That way you don't have to take the whole car back apart again. You can plug in your trailer or a tester like the one we've got here. You can pick a tester up at if you'd like to get one. You want to make sure you have your left turn signal, right turn signal, tail lamps, and brake lamps. With everything working properly we're ready to put the back of our vehicle back together and hit the road.That completes our installation of Tekonsha's universal four pole wiring kit on our 2015 Mercedes Benz M Class ML350.

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