Trailer wiring short and the black hole

[David Schwarz]

I have a 1992 F-250, and the running lights for my trailer are out. I've traced the wiring harness, and I have 12V for the running light circuit clear up until the harness disappears between the bed and the rear fuel tank. When it emerges on the other side, it's no longer a brown wire, but black with a white stripe (this wire feeds the 7-way plug for the trailer). So I'm assuming that somewhere above the fuel tank there is a harness connection that I cannot see.

A few months ago, I had a problem with the truck one morning. It started fine, but the voltmeter was twitching erratically, and the truck would fall on it's face when I tried to turn on the lights. After ten minutes of driving, everything was fine. I'm guessing that this was when the short developed in my harness.

The tank is full right now, so I don't want to drop it. However, I need to fix the wiring problem before a big weekend of racing comes up next week. For now, I'll just bypass the circuit so that I have a hot lead for the trailer. However, I'm curious to know what happens in the black hole above the fuel tank that I can't see. Any technicians (or experienced do-it-yourselfers) out there that can help me out?