I recently purchased a 2004 Chevy Silerado 6.6L 2500HD and when I went to plug my work trailer in, the trailer lights were not working properly. I noticed the 7-way had some corroded pins, so I purchased a new one. Now all the lights and blinkers work properly, but NO running lights. I checked all the fuses under the hood and they are all good. I tested the voltage of the brown wire when my truck's lights were on and nothing.So I have a feeling its in the factory wiring harness? Any suggestions?

 

TRL Park fuse underhood gets power when park lamps are on. If fuse is 12 volts with park lights on you have a open circuit in brn wire. Seen some rodent damage ender fuse block underhood many times.

 

Start checking the truck with a circuit tester. This circuit is often used for trailers with electric brakes. If your running lights are attached to it, then that is the problem. Use your circuit tester and check each wire with the working truck connector plugged in and find out which wires on the trailer are carrying the signals back to the https://www.carid.com/trailer-lights/