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I have a 7 pin harness and when i hooked up my 19 foot travel trailer this year the lights work except when i hit the brakes they all go out when i turn on the left or right signal they all flash
Something I learned a long time ago (and continue to re-learn) is that virtually all stupid electrical problems with trailers boil down to a bad ground. Your problem has bad ground written all over it. Check the white wire(s).