Hello all. Its been a while since i have been on here and thought i would ask for some advice on trailer tow issues. I am having a problem with the marker/running lights working on any trailer i pull. We have several here at work and i have hooked up to all of them and none of them work with my truck. Other trucks have worked fine with them. The turn signals and brakes all work fine, its just the markers. I have checked the square fuse with the clear top that relates to the Trailer Tow (TT) marker circuit and i even swapped the relay with the new one that comes in the glove box, since they had the same part number. Im at a loss and dont know what to do. Any suggestions?

 

Start but checking voltage at the plug end in the truck.
Also check that the fuse has power going into it.
Then check that the power is making it from the fuse to the relay.
After the relay you will have to find a wiring diagram to see where the wires run and see if there is a break in it.

 

My 03 lost power to the "trailer running or marker lights" turns out there is a fuse for them, I'd start there. If it's blown, check your trailers wiring to be sure there isn't a short or you'll keep replacing fuses

 

Had a similar issue with my RV and it was just a dirty connector on my truck. Cleaned it with some electrical contact cleaner spray and solved the problem.