Truck/trailer electrical connection

[bassfisher]

My trailer running lights are not working, but everything else works fine. Blinkers and stop lights work fine on the trailer, and all three work on our trucks, running, blinkers, and stop lights.

I little history, both of ours trucks worked fine on my 1993 boat, and 2004 trailer, but for what ever reason, on the 2002 boat and trailer we both start having running light issues. Hers (05 Chevy) started from day one right after we left the dealer I bought the boat from. They worked fine at the dealers lot, but about 5 miles down the road we lost the running lights only. We get home, and hooked up my truck (03 Ford). The runnng lights work fine. So I headed out someplace a few days later, and losts a few trailer lights, including the motor bar lights. We replaced them, and it's fine again. (The running lights still don't work on her truck when hooked to the trailer) I use my truck about 3 times, and the trailer running lights work. Well this last time I went about 2 miles in my truck and lost the trailer running lights only, and they have not worked since. I bought a tester and the running light indicator does not light up on either truck.
What could be wrong? Is there a fuse just for the trailer running lights someplace? Thanks in advance for any and all help!!