I am having problems with my vehicle-side trailer wiring on an '05 GMC Sierra 1500HD. It has a factory installed trailer wiring harness. I have no right side stop/turn at the truck's 7 way conector. I have tested with a meter and everything else works, so it's not a bad ground. There's just no voltage coming down the wire for the right stop/turn light. I need to find the other end of the trailer harness to test it there. Where does this factory installed trailer harness plug into the truck's main harness? BTW, all lights on the truck are working properly. Any help is appreciated.