I'm stuck! I'm trying to solve a similar problem, and get my trailer ready for camping in two weeks.

2002 Ford F250 7.3 crew

Checked out the trailer wiring by connecting a battery charger to each of the appropriate wires in the tow cable. Brakes, turn signal and running lights all lit up just fine.

Now turned my attention to the truck. Put a 12 volt probe on each connector contacts. turn signal (pin 5 & 6), brakes (pin 2), 12 V aux (pin 4), etc.. all work fine when I probe the connector on the back of the truck. However, there was no power on pin 3 for the running lights.

All fuses are good: Checked fuse 38 ("Trailer tow park lamps and backup lamps"), fuse 115 ("trailer tow battery charge") I'm thinking of buying a new relay 305 ("trailer tow battery charge relay") for ~$16 just to try something. I did have to replace fuse 115 a couple months ago when I first pulled the trailer out of storage. Also was having intermittent brakes, but found that one of the ground wires wasn't making good contact, but that's now fixed.

One thing that's odd is that I probed the socket for fuse 38, and it always has 12 volts, even when the lights are off.

Any ideas would be greatly appreciated.

Bob