Trailer light problems can drive you crazy! 
I have found that almost EVERY problem with trailer lights can ultimately be traced back to:
A) Poor grounding and/or connections.
B) Improper plug connections ( prongs, mud, rust, wiring, insulation)
C) Broken wiring.
D) Aluminum wires can be a problem after a few years of age.
It is almost a way of life that if you have lights of any kind on trailers you are going to, at some time, have trouble with them.
And you just have to patiently ( :shock: ) re-trace and follow - re-trace and follow, and a few 'few-letter' words won't help you find the problem, but it usually makes you feel better!
DOC HARRIS