So i am trying to wire up my 1987 s10 that i will be towing with my 1998 blazer for trailer lights. Im going to be using the lights on the 1987 s10 the trailer lights. So i bought a long 4 pin trailer wire harness and it has green,yellow,white, and brown. Brown is brake, white is ground, yellow and green are turn signals. So on the 1987 s10 it has light green,dark green, yellow, and black. black is ground, dark green and yellow are turn signals and light green is brake. Everything is working fine but the tail lights. when my head lights on my blazer are on the tail lights dont come on the truck. Brake lights and turn signals work just fine. Any one got any ideas whats wrong?

 

Brown is usually tail lights. The turn signals/ brakes are the same deal. Just when you hit the brakes it lights up the yellow and green together