My lights on the horse trailer will sometimes cross over

[Paulablair]

Sometimes when I go to hook up my horse trailer the signals are crossing and the side lights will flash on both sides. I jiggle all the wires I can find and after I am totally baffled they will suddenly work correctly. I have had it in to the trailer place 3 times and of course it's working correctly when they look at it. What do you think is going on?

Truck is 2007 GMC Sierra Duramax diesel.

Trailer is a WW.

[00Silverado4x4]

Sounds like you have a short somewhere. I'd start at one end and go through every inch of the wiring until you find a crossed wire. Also double check all grounds for corrosion and ensure they are tight.

[txab]

Check for good grounding for starters

[crankman]

Check for good grounding for starters

This above, when ever something weird goes on with my trailer lights I startwith ground path (white wire). Make sure its bolted to trailer frame clean dont use trailer ball for ground path IMO