A trailer brake controller has four wires. The ground and constant 12 volts are pretty easy. The other two wires are a brake light input and the brake controller output. Older Jeeps didn’t come pre-wired for a brake controller from the factory, so I had to run a new wire all the way to the back of the truck. The brake trigger in a Jeep with red turn signals is the tricky part. If I just hooked to the brake light wire, then when the hazards or turn signals come on the trailer brakes would as well. That’s probably not good.