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

 

Either there is a dirrect short in the wiring or there is a poor ground...I would start with the ground.

 

sounds like the trailer ground is bad.

 

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).