it sounds like the wires got crossed somewhere in the main front to rear harness, usually when someone splices in a trailer harness, best bet is to download a wiring diagram for your model truck and trace the wires from each plug (one that plugs into the firewall and the other end ties into the rear tail light harness above the rear bumper-under the bed any way's) these plugs have four colors, light green, yellow, dark green and brown, I'll bet whoever did the splices just wasn't careful enough to get them in the right order.
The rear tail light harness is the one that seems to get hacked up more than any other part of the wiring. I was looking for a good tail light harness for my build at a local yard, and of course, couldn't find one. I looked at a clean 1980's model Chevy truck, and the plug that ties into the harness from front to back had the same plug on it (four pins kinda like a "T" ) any way's, the sockets were different, but essentially the same harness, I just cut off the newer sockets and spliced in the factory early ones. hope this helps....
Ben
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