Wiring humdinger for the brave
 

One thing I have noticed with my truck is everytime I pull a trailer with electric brakes the trailer brakes come on while backing up. What I didn't realize was that everyone o put the truck into reverse the brake lights come on as well. I knew there was a problem but just figured it was in the 7 prong connector. I figured I would tackle that today. I opened up the connector and huh. Looks pretty good inside. Plus it isn't wired to have backup lights into the plug. That wire ends by the fuel tank. So I start to unravel the electrical tape and come to a bundle of twisted together and taped up wires. I shuffle them around and alas the brake lights are off. I went under the truck to find my problem only to find that the wire going to the passenger side brake light was now disconnected. Hook it back up ant the brake lights go back on. Disconnect it and they go out again. Ok the problem must be in there right? Now here's where it gets interesting. If I remove the trailer backup light relay my truck lights work properly. Plug it back in and the brake lights are back on. Another thing I noticed. When hooked up to the trailer to stop the trailer brakes work flawlessly. But I according to my multimeter there is a constant supply of 12 volts to the trailer brake pin on the truck, whether it's in reverse or not. Anybody have any ideas ?

Interesting situation you have. IIRR the trucks that came with the trailer tow package, had 2 plugs that ran to the back of the truck. One had 3 wires, 2 for the turn signal, and the other was for the marker lights. The other had 2 wires, 1 was hot all the time with the key in the run position, which was for the trailer battery charge, and the other wire was for the trailer back up lights. I don't recall the wire colors though. It sounds like the constant 12 volts you have going to the brake pin is the battery charge wire, but if that was the case, the trailer brakes would be on all the time. Not much help, but here is a bump to the top.

My brain is fried..sorry bub.I'll look tomorrow...

Yup. It's strange. And like I said if I remove the trailer backup lift relay or disconnect the passenger side brake light at the junction the brake lights go out. The brake controller seems to sense of the trailer is hooked up and I wonder if that's why it has voltage at the plug. Just a thought. I'm gonna think about it a bit. Then I may bring it to a friend. He's pretty good with electrical gremlins and I have faith in him.

Factory wiring colors for 7 pin trailer connector ...

Yellow - Left turn lamp
Dark Green - Right turn lamp
Black/Lt Green - Backup Lamps
Orange - Battery charger
Brown/White - Marker lamps
Dark Green - Electric Brakes
White - Trailer Ground

Notice there are two Dark Green wires.

Interesting. That seems to make sense on where all my troubles could be. I bet if I find the trailer backup wire it will have power when I press the brakes. Thanks. That seems to make a lot of sense.