99 yukon

Look in the glove box, there should be a fairly simple looking pigtail wrapped up in the glove box unless someone has removed it, usually it is still in the glove box even in an older truck because no one knows what it is for. If the harness is still there simply unwrap it and look to the left of the break pedal, near the side, on the floor, there will be plastic cover shaped like a small box about 3"x3". Remove the cover and there will be a couple empty plugs, simply plug the wire in where it fits. route the wire to your new brake controller and connect the four wires. The instructions will come with your new controller to tell you what color goes with what color. If the harness is gone you will have to got to the dealer for a replacement, should be about $20.