I think its fuse #46 or 47 15 amp if I remember right, had the same problem and kept blowing the fuse come to find out there was a grounded hot wire in the harness on the trailer.

 

If it's not the fuse, it's probably the PDC ....... and no, they aren't cheap.

I had to replace the PDC on my dually because the running lights quit working. I pulled all of the fuses and relays out of the old one and plan on opening it up someday to inspect the relays built in. My wife's srw now offers no brake/turn on the right side of a trailer and as soon as I get around to opening the old PDC, I might be able to fix hers.

 

Yep, a modulite is under $50 last time I checked. Excellent alternative.

 

When the running lights quit on my truck, the PDC was dealer only, expensive and on national back order. If the only issue is the trailer, this is a great alternative! If I can't repair my wife's, I'll probably look into the modulite, hers is a trailer only issue.