I have 2002 GMC with 7 pin plug. my utility trailer has a 4 plug. i bougt an adapter with LED in it. i plug it in to the trurk then pulg in the trailer. when i test the light on the trailer, the running light work ok. when i turn the right turn on both right and the left comes on. The side marker allso blink too when the right or left trun blinker is on. The trailer was rewired in 98, new plug and light, all of the connection where solder and taped. there is a grund wire to all of the lights

Thank you

I've never had your problem. But adapters are an "iffy" solution in any case. More connections to corrode and fail. I would suggest you purchase a 7 pin plug to match your truck. Then wire the trailer to the diagram that's on the cover of the truck's trailer connector. (I'll assume the truck has the factory towing package)