I have a 03 GMC 1500HD. I am putting a flatbed on it. NOT using LED lights.
My problem is on the pickup their is a wire for brake and a wire for stop/turn. My lights on my flat bed hand one wire for brake and stop/turn.
So I wired the 2 wires on the pick up together. Well I am getting some back feeding. When I push on the brake the turn light in the mirror lights up and when I turn the blinker on all the lights blink. Even the brake like on the cab.

So is their some thing than will prevent the pack feeding? Wire in a diode?

 

I would separate the wires you wired together and add a small light to each side of the flat bed and leave as it was intended to be on the truck.