Trailer Wiring

Thanks for all the suggestions and thoughts! Yes, I do have a brake controller - I have a Drawtite Series two box that should handle 6 or 8 brakes. The last time I towed one of my trailers, I was told that my running lights were intermittent - probably the wirenut that was missing. The trailers that I'm having the most trouble with belong to the Band Boosters for my sons High School = Oakmont High School. However, no one else that pulls these trailers seems to be blowing fuses. I did get an adapter for the 7 plug to flat four to see if it was the flat four or maybe the RV 7 blade - it didn't seem to be a difference. I was wondering if something was loose in the RV 7 blade that when I used it one of the blades would move and short out with a different circuit of ground, but that didn't always happen. I think I have an intermittent short in the wiring - near but not with my splicing and was trying to find a way to isolate the trailer from my van because currrently when my fuse blows, I don't have brake lights anywhere!
Again, thanks for all your thoughts!