Trailer Wiring Harness Installation - 2018 Mazda CX-9

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How to Install the Trailer Wiring Harness on a 2018 Mazda CX-9

Speaker 1: Today, on our 2018 Mazda CX9, we're going to be taking a look at showing you how to install the Tekonsha T-One vehicle wiring harness for factory tow package with a four pole side trailer connector. That's part number 118278.Here's what our wiring looks like once we have it installed. I do want to mention we did use a bracket to mount our wiring more permanently outside by our hitch, because, traditionally, our wiring is meant to stay on the inside of the vehicle. Our wiring is going to provide us with the four-pole flat trailer connector to get us all the lights we need to safely and legally get down the road with our trailer, like our tail lights, turn signals, and our stop lights.Our wiring is going to plug directly into an existing plug that's on our vehicle already. Our wiring's also going to have a converter box to convert those signals into working signals to go out to our trailer. Also, to protect our wiring and our vehicle from any kind of back feeding or problems that may arise on the trailer end.

Since our trailer was designed to stay on the inside of the car at all times, typically, you would drop it out the back of your hatch, close the door and your wiring would be hanging down right here. Now, a lot of people don't like the idea of that because they think it's going to get damaged because of the hatch closing on it. Well, that's typically not going to happen because it does that have that weather stripping where it's going to have a little bit of give as long as you stay away from the latch mechanism itself it will be okay. But a lot of people do like the idea of being able to mount it out here for a more permanent solution. In our CX-9, if we we're to remove our spare tire from the bottom of the inside of the hatch here, there is going to be a plug on the bottom of the floor that we can route our wire down and have it come right by our receiver too.The four-pole bracket that we're using here is sold separately and you can find it here at

Our hitch has this bracket we could use to mount our four-pole bracket to, however, if yours doesn't, we have a wide variety of brackets to mount any of your electrical accessories up to your hitch here at So, whether you decide to leave your wiring on the inside or route it to the outside, we're going to have plenty of wiring to get everything connected. Let's show you how to do that now.To begin installation, you're going to want to open up the rear hatch on your CX-9 and then we're going to come and we're going to remove all the floor coverings that are in there and then we can pull the carpeted section underneath that where our spare tire is.Now, we're going to want to pull all the panels out, including the ones on the side. We'll pull those out and these plastic compartments, we just come underneath and lift up fairly hard. Should be able to pop them out of the clips and we can set these aside and you're going to want to take out the one on the passenger side as well.If when you removed your panel, the plastic clip got stuck in there, you can take a trim panel tool, works the best, or a flathead screwdriver, if you don't have one; you're going to want to come underneath and pry it out.

Once we have it removed, we can just slide it back into the panel so that when we put it back in, it'll go back into place.We can see the clip was pulled out from here, just want to make sure the bottom phalange is on top of that ridge, we can press it back in. This will make it easier for us to install later. We're going to repeat that same process on the passenger's side.With those panels removed, we're going to remove the threshold at the very back here. If you start off with the corner, and you start lifting up, you're going to want to work your way from one side going across till all the clips are removed and then we can set that panel aside as well.We're going to have to loosen up this side panel as well so the very corner underneath where our threshold was, we're going to have this pushpin fastener. Now, if we come underneath the center section, we're going to pop out the middle of it first, with the trim panel tool or an extremely skinny flatblade screwdriver.

Pop out that center section and then we can pull the rest of the pushpin out. Then if we move farther in to where our cargo hook is, we're gonna wanna pull that out and the cover should come down a little bit and there's going to be a bolt that's holding it in place. So, then you're going to have a 10 millimeter socket and we're going to pull that whole assembly out. Now, if that plastic piece isn't coming out, that's okay, you just want to make sure the bolt's out because there is a plastic retainer that's holding it to this larger panel here.You want to start at the bottom corner and kind of work your way up and there's going to be some clips running along the top edge here. You're just going to want to gently pull, releasing those clips so that we can get access behind this panel. With the panel loosely pulled away, we can kind of reach back and push on that panel and release that plastic clip that was holding the rest of the tow hook in.Further up on the panel towards the plastic section, we're going to have another cargo hook; we lift it up and we'll have another bolt holding it in, pull that one out, pull the assembly out and that'll give us a little bit more room so we can see what we're doing behind the panel. Again, you don't want to pull too much because you don't want to cause any damage to the panels here, but behind the insulation here we're going to be looking for a connector that's going to hook up to our wiring harness. So our connector is going to be pretty far back, but if you can see it's going to be that white connector that's connected to the other one with the several wires coming out of it. We're going to take our harness and we're going to be plugging it in right here. We're going to take our wiring harness, we can see the plug is going to match the one on our vehicle, just want to pay attention before you get in there so you can make sure that the locking tab's in the correct position because it is pretty hard to see and get to back there.I reach in, pull the connector in and if we follow it back a little bit, we'll notice that there's going to be a bolt holding this other connector in place and that's what we're going to use for our ground. Now, the ground wire is going to be the white wire with the ring terminal on it so we'll grab a 10 millimeter socket, remove that bolt and slide the ring terminal in place. We can take that bolt, slide it through our ring terminal, then we want to make sure we put that connector back in before we put that bolt back in and we'll get it started by hand so I don't have to worry about anything falling down. We're going to take out 10 millimeter socket again and snug it up.Now, to mount our converter box, we have a few different options. We can use the outlet at the top and take a zip tie or two and secure it to some wiring or some other place back here or we can take the provided double sided tape and remove one side of the backing and stick it directly to my converter box. Then we can find a nice flat spot that's not going to interfere with anything and we can stick the box directly to it. Just want to make sure you have enough room for that box to be wherever you're going to put it once we have the panel back in place. Remove the other side of the backing and I'm going to actually mount mine pretty low, down here on this metal panel; that way I know it's not going to interfere with anything. And just take your four-pole wire and make sure it's coming out the bottom of the panel. You're not going to go anywhere the pushpins are going to interfere with and for now we'll just set it towards our spare tire and we can start working on getting the panel back in place.Making sure all the clips go back in and the weather stripping's on the right side of our panel. Our cargo hook back in place. We can reinstall our lower cargo hook, making sure everything's lined up, putting the bolt back in and tightening it back up.Normally now would be a time to put the rest of your panels back in place and we would leave our wire right here by our spare tire; however, our customer has requested that we run the wire outside, close to the receiver hitch. So we're just going to move it aside for right now and we're going to need to remove the subwoofer and pull the spare tire out. So, if we come to the top, we're going to have a 21 millimeter head bolt, so we can take a socket and remove it. You can also use the tools that are in your spare kit because it will be the same size as your lug nuts so you can take the wrench out of there to remove this as well. Pull the bolt out and set it aside. Just so our stuff doesn't get damaged, if we come to the left hand side towards the drivers side, there's going to be a connector here, if we push in on that tab and pull out, we can pull the connector out and we don't have to worry about pulling on those wires too much. We'll lift this up and very carefully just set it aside so it won't get damaged. Then we can pull our spare tire out and the foam insulation underneath.With all that removed, that's going to expose these plugs here that are on the bottom of trunk pan. Go ahead and pull one of them out; I'm just going to go ahead to pull the one that's closest to the wiring out and then we can take our four-pole connector, feed it through. It is going to be a little bit of a tight fit but it should fit through there; just a little bit of pushing and we're going to push it towards the hitch. We're going to run as much wire out as we can; have a nice straight shot.This is going to be optional but I'm just going to take some tape and I'm going to cover my wire and try to get it to be as flat as I can against the floor, that way I don't have to worry about it moving or my spare tire causing any damage to it. Keep it nice and secure. Then once that spare goes back on there it won't move around.Now, for that plug that we pulled out, I'm going to take a pair of scissors and we're just gonna cut a slit into it, make a little bit of room, that way my wire can go around it and I can have it go towards the center of the plug and then I can put it back in place.And just to help out with any kind of leaks, so I don't have exhaust fumes coming back up, take a little bit of silicone and seal that hole up that I cut in that plug. We'll let that dry and we'll move to the outside where the wire's ran.My wiring came out behind my muffler right by my hitch. Now, we have a few different options, we can either just take our dust cover and connect it to our safe chain connection point and leave our wiring out. Tie up the excess, however, our hitch has a mounting bracket right here and we would like something a little more permanent. So we're going to utilize this bracket as well as a four-pole mounting bracket and we're going to take the included hardware and we're going to drop it down through both brackets and then on the bottom side we're going to secure it with a locking nut. I'm going to get this one loosely started so I can get the other one in place.Since these are locking nuts, put them up top with a Phillips screwdriver and I'm just going to hold this with my finger while I tighten up that screw up top; make sure it's nice and secure. I'll do that for both of them. With the bracket securely mounted, if you look on our four-pole, we're going to notice they have these little ridges on there and that's what our bracket's going to use to keep it in place. Coming from the back side, just kinda work it in going back and forth till it hits those ridges. Now when I push on it it'll keep it locked in place.Now all we have left to do is take our excess wire and tie it up. Now, you could find a couple different places; you can go to the top of the hitch and use some zip ties, or if we look up farther, there's going to be a mounting bolt and a bracket that's coming off the back of the body. We could just take our wire, bundle it up and then use that to our advantage and tie our wire up here. I'll bring as much wire over as I can, then take another zip tie going through my bracket, tie my wires as high and out of the way as possible. Hopefully it'll hide them enough where we can't see them because they'll be behind the bumper here.Then we start putting our spare and all the panels back in place. Just take our connector to the subwoofer, make sure the tab is back on top, line up the connector, push it in and lock it down and replace that bolt.Line our threshold back up, make sure all those clips line up. Once we have it lined up, kind of go across making sure they all lock back into place. Again, just make sure you get your weather stripping on the right side of that panel. Take our side panels and put those back as well. You just want to make sure you line up those pins, make sure they lock back in. Put the lower floor piece back in as well as our side pieces, finally the floor covering that goes on top.So the final thing to do is test our circuits. I have my four-pole tester here that I can plug in. If you need one of these, you can pick one up here at using part number I26. Now I'm going to get a second pair of hands so I can run my lights so that I can verify they're all working properly. If I turn on my headlights, we can see that they're all working properly as well as my left turn signal, my right turn signal and my brakes. Now all we have left to do is hook up to our trailer and hit the road.That'll finish up your look at the Tekonsha T-One Vehicle Wiring Harness for factory tow package with a four-pole flat trailer connector, part number 118278, on our 2018 Mazda CX-9.

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Info for this part was:

Employee Andrew K
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Andrew K
Employee Kathleen M
Video by:
Kathleen M
Employee Jonathan Y
Video by:
Jonathan Y
Employee Jacob T
Video Edited:
Jacob T
Employee Chris R
Video Edited:
Chris R
Employee Cole B
Installed by:
Cole B
Employee Brent H
Test Fit:
Brent H
Employee Robert C
Test Fit:
Robert C

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