I am helping a buddy re wire his car hauler. I have wired a couple of trailers so he asked for my help. Trailer has electric brakes on both axles, it has the RV style plug. When you have your headlights or parking lights off, everything works perfect, but if you turn your lights on your truck, the trailer loses blinkers and on my truck my left blinker won't flash. Any ideas?

 

You have a bad ground or the wiring is crossed somewhere. Similar thing happened on my boat with the trailer when I bought it. Headlights on, no blinker. Turns out the previous owner had wired the side marker light to both the running light wire and brake/turn signal wire.