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My recommendation to everyone would be after purchasing a Frontier and installing a brake controller, always double-check all circuit operation with a plug-in 7-pin portable trailer tester before attaching a trailer. Too many random trucks in random years, including my 2014 Pro4, have had the wiring harness reversed from day one. I haven't towed with electric brakes yet, just surge brakes, but thankful that this forum and eTrailer.com both mentioned this as a possibility and I got it corrected in my installation BEFORE towing a trailer. One of the moderators might want to make this a STICKY in the new electrical section to warn new members up-front.
 
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2018 Frontier 2wd king cab
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I have a 2018 frontier I bought the t-1 connector kit from etrailer. My problem a year later now is my left brake light stays on all the time while the trailer is plugged up. It's just the plain flat 4 connector I have emailed etrailer it's been about a week now and they haven't answered with any possible solutions yet. I'm thinking it could be 1 of the 3 relays that comes in the kit but I don't know which one operates the brake lights? There are 2 blue relays, and 1 larger brown relay.
 

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I have a 2018 frontier I bought the t-1 connector kit from etrailer. My problem a year later now is my left brake light stays on all the time while the trailer is plugged up. It's just the plain flat 4 connector I have emailed etrailer it's been about a week now and they haven't answered with any possible solutions yet. I'm thinking it could be 1 of the 3 relays that comes in the kit but I don't know which one operates the brake lights? There are 2 blue relays, and 1 larger brown relay.
try unplugging them one by one and seeing what happens.
 

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Part of this is trailer plugs are wire differently if you trailer has electric or surg brakes. Surg brakes are wired with the blue wire is connected to back up lights, this powers a valve to by-pas the brake fluid to the master cylinder so you can back up a hill without blocking the slid on the hitch.
 

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2018 Frontier 2wd king cab
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No brakes just a flat 4 connector for running the basics. Turn signals, running lights, and brake lights. It has been wired this way for about a year now. The 1 brake light staying on just started a few days ago. That's what the peculiar part is. Why it worked fine for so long, and then started with the 1 brake light issue. One would think if it was a wire issue, or relay then both brake lights would stay on not just the left side.
 

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WJ, check all your trailer grounds. Are you saying that the left brake light comes on as soon as the trailer's connected, or trailer connected and engine on, or trailer connected and running lamps on?
 

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2018 Frontier 2wd king cab
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WJ, check all your trailer grounds. Are you saying that the left brake light comes on as soon as the trailer's connected, or trailer connected and engine on, or trailer connected and running lamps on?
Yes as soon as I plug it in, or leave hooked up while the truck is turned off. The left/ drivers side brake light stays on until I unplug the trailer. I hooked it up the other day so as to have it ready for the next morning. Later that evening my neighbor called and ask if I knew I left my truck lights on. When I went out to check it was just the left trailer brake light on the rest including the truck where all off. It's definitely odd for sure it's been wired the same way for about a year now, and it never did this before.
 

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2019 Pro-4X (30,000km)
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333937


Don't have much to add that others have not already...but here's a handy diagram I made for myself...

Uhm....I didn't see it above, but have you check the TRAILER wiring? Perhaps the truck is good, but the trailer is mis-wired?
 

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View attachment 333937

Don't have much to add that others have not already...but here's a handy diagram I made for myself...

Uhm....I didn't see it above, but have you check the TRAILER wiring? Perhaps the truck is good, but the trailer is mis-wired?
Been wired the same way for almost a year now. I even tried it with our boat today, and it does the same thing.
 

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2018 Frontier 2wd king cab
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I asked etrailer for help/ info seeing how they recommended everything I used to setup the trailer lighting. No answer yet so the issue remains unsolved.
 

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Still a mystery I have to take it in for a oil change soon. I will see if they can figure it out. I'm leaning towards something with the bcm.
 

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On the original issue, my 2016 Pro-4x was the same way. I ordered a Tekonsha 7 pin trailer plug for my truck, and noticed this problem after reading reviews between this one and the Curry plug. Before I did the install I swapped the black and blue wires at the plug, and it was good to go. I tested everything with a multi meter to verify as well.

There is no need to cut wires, just disassemble the plug and swap the black and blue wires. The reason this happens is because the original trailer brake wire was incorrectly swapped on the truck side to be the 12v charging wire, so it is always hot.

When I did my install I used a 2 inch hole saw and mounted it in my bumper so it wasn't hanging low to get snagged on rocks.
335349
 

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I have run into issues with this problem, mainly due to the difference in aftermarket (i.e. Curt, Reese, etc) as opposed to factory Nissan 7-pin wiring harness, Problem is a lot of the aftermarket 7-pin plug and play harnesses end up reversing the trailer brake and the 12 volt positive accessory pins.

Just traded my 2019 Pro-4X for a new 2022 Pro-4X. 2019 did not come from the factory with the hitch and 7-pin wiring harness installed, so the dealer went to a local hitch supplier for the install before it picked it up. However, ended up switching the trailer brake wire and the 12 volt positive accessory in the junction box on the trailer as the brakes were continually on with the ignition key in the run position Had to switch the wires back in the junction box again when I picked up the 2022 model.

As per the wiring standards diagram for a 7-pin female connector, Nissan has it right on the 2022 model from the factory.
 
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