..the white wire on the trailer harness is normally used for grounding...it sounds more like a short in your blinker/brake light circuit...does a fuse blow for both side turn signal or just on one side?...do you have a meter to check resistance with or some other sort of circuit checker??? If so, check the resistance/continuity between ground on your trailer and the yellow/green/brown wires on the harness(there should be infinite resistance or no continuity between them)...there is a very slight
possibility that the circuit is being overloade by the addition of the trailer lights...if you are grounding everything back to the frame, be sure all points of contact are clean of rust, paint, corrosion, etc...
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