Ok, this problem isn't on a Ford vehicle, so I hope I don't get in trouble asking for help here. I figured everybody here is very knowledgable and my best bet at getting help. I drive a '93 GMC Topkick truck that has a fuel tank on it. The lights have been working great, until last week. I had a socket go bad and it was crossfeeding between the turn/brake and the taillights. I was installing a new socket, had the tail lights and turn signal on so I was sure to get the wires right. Was working away and looked up and had no taillights/marker lights on the body. Still have the marker light on the cab however. Messed with it for about four hours and couldn't figure out the problem. Tried circuit breakers, and swapping relays, to no results. Did some checking, with the body lights unhooked from the feed wire coming off the truck, had 12.6 volts to the feed. Hook the just one marker light back up and the meter doesn't show any volts, yet markers on cab still work. Did a continuity test from the non powered side of the circuit breaker, worked like it should. The turn signals and break lights all work on the body, just no taillights/markers. Did check to see if they were working by hotwiring direct to a battery, they all worked. The only thing I didn't try was putting a fuse in the hotwire to see if something would be grounding out. I've always enjoyed problem solving wiring on vehicles, but this one has me stumped. Please help.

 

You got a what a a a a what the heck are you doing here, I am gonna bottom kick you outta here!!!!!! Just kidding... Double check all the grounds on that circuit you could just run a temporary one straight from the batteries and see if it will work. It sounds like the original problem was a ground problem too, If a light looses ground it will try to find one even if it has to go through another light, that's why when say you turn on a signal it could go thru the reverse and light up that light to use it's ground!!

 

The original problem was a crossfeeding issue through the contacts on the socket. The contacts are setting in a rubber insulator and it was shot, so the contacts were touching. All the lights on the body are grounded through the light housings to the body, no ground wires anywhere. I know that electrical problems can usually be traced back to a bad ground. I am planning on trying to run a fused wire direct to see if it blows the fuse. If not, I'll wire it up with a seperate switch for good and mark it up to being a problem in the cab and mark it up as another bad GM experience.

 

I know what you mean about the melting sockets. I use ford pigtail sockets with the ground wire to replace them instead of replacing the whole light. I do the same with boat trailer lights, It is a heck of alot cheaper then gettin a whole new housin every time they corrode out!! I think it is $3.00 for the ford socket with ground wire.
I would run a ground wire back to them just to see if it cures it, If that fuel body isn't grounded correctly and some static builds up or there is sparks from a shoddy ground connection well you know the possible outcome!!