My rig is a 1993 F-250 with only 40,000 miles on it. I tow an Airstream trailer and all of a sudden my running lights don't work. I Applied 12 volts to the D prong on the trailer and they lit. I checked the D prong on the truck and have zero voltage with the truck lights on.Thisis a factory wiring set up. I looked in the fuse box but couldn't find one marked for the trailer. Does anyone know if it is a fuse or circuit breaker that controls these lights. All the lights work perfectly on the truck and all the rest of the lights work on the trailer.

Doug

 

You might just want to check all light related fuses.

Also, try pulling the headlight switch out and see if you have power then.

I don't know if my trailer wiring is factory or not [I don't think it is] but I have 2 circuit breakers tied into the trailer wiring that are mounted next to the starter solenoid. You might want to look in that general area; or maybe trace wires from the solenoid/battery to see if you can find any inline fuses.