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Before I go tearing off into this. I have a 1998 Ford F-150 4x4 with 5.4L and factory tow package. I plugged in my trailer and had no lights. No brake lights, no turn signals, no tail lights, nothing. I checked at the 8 pin plug on the back of the truck and the ground reads to ground. 12V+ reads twelve volts, but all other pins read zero volts even though all the lights work on the truck. I pulled the 8 pin connector apart hoping that it was just corrosion inside of it. There was some so I cleaned everything up and put it all back together but the symptoms remain. Is there another connector further up the frame that I need to check? Set of relays?
I checked my manual but it does not have a wiring harness schematic that shows a trailer wiring harness.
If the fuses are good, follow the wires out of the back of your plug, it should lead you to a big T connecter mounted to the body, this is where the trailer harness is spliced into the system. It will have plug ins on each end of the T. You can get a replacement from Ford or most part stores.