I just bought a '99 F-350 V10 Auto 4x4. The only working lights I had were headlights, brake lights, and turn signals. After replacing the missing #8 mini fuse under the hood, (Trailer Tow Electronic Brake Illumination, Park Lamps, Trailer Tow Park Lamp Relay Coil) I had running lights and dash lights, the dimmer and cargo light also worked. After turning off the switch, the headlights shut off but the running and dash lights did not. The only way to shut them off was to pull the #4 mini fuse(Trailer Tow Backup Lamps, Trailer Tow Park Lamps).

I pulled out the headlight switch from the dash and it seems fine. With the running lights stuck on, I disconnected the switch from both wiring harnesses and the running lights were still on.

Could this be a bad parking lamp relay? If so, where is it located? I've read that it's behind the dash by the radio.

Or should I tear apart and redo the cobbled together trailer light setup which was converted from a 4-flat to a 7 blade hookup on the bumper and a 6-pin in the bed.

 

You could take the trailer wiring apart. It sounds like the running lights are getting power from somewhere they are not supposed to, and if they have a trailer wire crossed up, that could cause it. If you find a main harness plug going to the rear, just unplug it and see if they go out(the front lights, the rear ones will obviously go out). Just like you pulled the headlight switch out and eliminated that part of the circuit, you can unplug different parts of the truck and narrow down where the problem is.

 

Thanks, that's exactly what I did this afternoon. After unplugging the trailer light harness everything worked as it should. I wired in a new 7-blade plug and everything still works, including the lights on my trailer.