My truck lights work fine, but my trailer lights dont. The running lights and turn/brake lights are trying to use the same circuit on the trailer. Its not the trailer because i have the same problem at the hitch connection.

If I run just the running lights it is dim and if I use either turn signal both blink very dim, and if I apply the brakes everything quits......

Trailer works fine with another truck, its my truck

 

you lost your ground. looking at the trailer plug it is the first pin clockwise from the key

 

2nd that carl48!