Tekonsha T-One Vehicle Wiring Harness Installation - 2021 GMC Terrain

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How to Install a Tekonsha T-One Vehicle Wiring Harness on a 2021 GMC Terrain


Hi there, GMC owners. Today on your 2021 GMC Terrain, we're going to be taking a look at and showing you how to install Tekonsha's four pole flat trailer connector vehicle wiring harness. And this is what our wiring looks like when it's installed. We did mount it into a four pole bracket, which doesn't come included with the wiring, but you can get that here at etrailer. And there's plenty of mounting options.What you do get though, is your four pole flat trailer connector and everything you need to hook it up. Our four pole flat trailer connector here is going to provide us with all the necessary lighting signals that we need for our trailer to keep us DOT compliant in all states.

This includes our left turn signal, right turn signal, tail lamps, and brake lamps. And you can see here that it's mounted outside the vehicle, so it's always there when we need it. And it has a dust cap that comes pre-made onto it to help keep out any dirt, debris and moisture to ensure a long lasting connection.Since this is a plug and play harness, and it works with the factory tow package on your terrain here, it's an extremely quick and easy installation. There's a couple checks to make sure you've got the proper relays installed. Those do come with the harness.

So if they're not installed from the factory, you do have those available with your kit.And then it's as simple as plugging in our harness. We're going to show you where to find, if those relays are installed, and where you're going to plug in your four pole connector here. So follow along with us and we'll get this done. And it should only take you a few minutes to do it at home.We'll begin our installation here at the back of the vehicle with our lift gate open, by accessing the fuse box here in the back. There's a couple of relays that we're going to check to see if they're installed.

Sometimes they're installed from the factory and we don't need to do anything. Sometimes they are not.You'll have two relays that come included with your kit. So we're going to see if they need to be installed, or if we got them from the factory. The fuse box covers located right here on the driver's side at the back. We're just going to get our fingers underneath there and just pull that cover off.

It just comes right off of there.Now that we've got our cover off, we can see a row of relays here at the top. Now we've got a relay installed. This is for the trailer signals right here. So we don't need to install the relays that they come with. The spot here is number three. We're just going to go ahead and pull it out so that you can see there how it goes. You can see this one's got slots in it.These are the other two relay spots. These are blank on our vehicle, but you may have a flat surface that looks just like this. And if you do on slot one, you can also insert a relay in there. They're both for the stop/turn circuit, so we only really need just the one here. But since they do come provided with extra relays in your kit, if you have the slots available, you can plug these in. But as long as you've got this one, you should be good to go. If that one's missing, just take one of the relays that comes in your kit and plug it into that spot. We're going to use the factory one since it's there.We're now underneath the vehicle. And where we're looking now is the gap between our rear fascia and our hitch right here. If we look up between this gap, you're looking at the bumper beam there. Our connector that we're looking for for our wiring is located right there behind the bumper beam. You can see the connector there. It's got a dummy plug in it and it's just that little red tab that's the lock tab. That's what we're looking at there.We're going to go ahead and disconnect the dummy plug that's in there. To do that, you're going to pull the lock tab away from the connector. Then you can press in on the release tab and pull that off. I'm going to try to let you guys see it here, but it's a pretty tight spot so my hand might get in the way.So here we got the dummy plug that I pulled off of it. I pushed the red tab that direction away. That'll expose the release button there that you press on. And then we can pull this whole thing off and we don't need it any more. You can throw it in your glove box or discard it. We're going to be plugging in our new connector, right into that factory plug.We're now going to take our new harness. This end is going to plug directly into that factory connector where we had removed our dummy plug. So just kind of get up in there. And it's going to be tight due to the hitch being here. And we just want to make sure that they click together. Make sure you can't pull them back apart. Heard the click. Give it a nice tug.Looks like we're all good. It's not going to come off. Now, you can see you have quite a bit of excess hanging down here. We can zip tie this excess up. And what I often like to do is take the four pole in here and you can wrap it around your hitch on the safety chain loop, using the dust connector there, the little dust cap.Now we can take this and we can secure it up. One of the things that we're going to be doing now, our customer purchased a bracket for his four pole. So while this is a great option, if you don't have a bracket. If you do have a bracket, we've got plenty here at etrailer, we're going to be installing a bracket and mounting it in the bracket. Because if we take our wire and mount it up now, we might not have the length we need to get where our bracket is.So I always like to choose the location where we want our connection to store first and then tie up the excess. So here we went ahead and mounted our four pole bracket. You can purchase these here at etrailer. They do come separate from your kit. We just removed one of the factory bolts that was at the bottom, and then bolted the bracket up with that factory bolt. And then we just took another screw and just ran it right into the plastic to keep it from pivoting.Our four pole then, can just push right into place through our bracket. And it can be a little tight getting it in there, but we want it to be tight because that's what's going to keep it from pushing back out. Here we go. Then you can re-install your dust cap to protect your connection. And the rest of this wiring, we're just going to then zip tie up behind our fascia there.All right everything's installed. We want to test it out to make sure it's working properly. We're going to plug in a tester here at the back and make sure that we've got all of our signals. You could also plug it into your trailer at home.I do highly recommend the tester over your trailer. In the event that you have any faults on your trailer, you might get a bad reading and you might think there's something wrong with the wiring on your vehicle, but it ends up being your trailer. A tester like this can help you isolate that issue and ensure that your vehicle is working properly.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 load up our favorite accessories and hit the road. And that completes our installation of Tekonsha's four pole flat trailer connector vehicle wiring harness on our 2021 GMC Terrain.


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Info for this part was:

Employee Joshua S
Video by:
Joshua S
Employee Jonathan Y
Video by:
Jonathan Y
Employee David F
Video by:
David F
Employee Aidan B
Video by:
Aidan B
Employee Jacob T
Video Edited:
Jacob T
Employee Chris R
Video Edited:
Chris R
Employee Cole B
Installed by:
Cole B
Employee Robert C
Test Fit:
Robert C
Employee Ryan G
Test Fit:
Ryan G

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