Running lights on the truck work but not to the trailer plug. No fire at the factory plug under the truck. I've checked under the hood for a bad fuse. Are the truck running lights on a different circuit than the trailer plug? Everything else works fine (Trailer blinkers, electric brake). Maybe there's a fuse for the trailer running lights I can't find. In previous trucks, when the truck lights didn't work, neither did the trailer...or the dash lights, so I assumed they were on the same circuit.

 

Need to know what year the truck is. The newer CAN bus systems have done this and a reset by the dearler was needed to correct the issue. Do a search on here, been dicussed many times before.

 

What Spooler said, plus, follow the wiring under the truck from the rear bumper, see if it's pinched between the bed and frame. Had this happen on my truck, all sorts of heck broke loose with my lights.