I have a 2004 f-350 psd. i tow a few trailers with it and of all of them the running lights work just fine, however the turn signal and brake lights arent getting any juice. I checked the fuses under the dash and i checked to see if all the wires were in one piece and all seemed fine. ill take any advice i can take. Thanks

 

This info is from a 2003 superduty diagram, so I hope it's the same.

The left and right turn come from fuse #6, 20 amp in the underdash fuse box. The power leads to a aux relay box #5, which is located under the hood, on the driver's side near the firewall.

The running lights for the trailer come from fuse #38 in the underdash fuse box, go to a trailer tow relay parking lamps, which is built into the fuse box and you can't get to it, and then leaves to go to the rear on a brown/white wire.

It's unusual that both the turn/brake and the parking lights are both out. I would make sure everything is plugged in at the rear.