Hooking up the trailer brake controller on a GMC pickup is a relatively simple process. 12-volt power is routed to the controller from a source in the engine compartment, and the signal to the trailer's electric brakes is routed from the controller to the round seven-pin hitch. The other two wires are connected under the dash close to the location of the controller. In fact, some GMC pickups are factory-fitted with a connection harness that makes the hook-up a "plug-and-play" job that should take no more than a minute or two.
Consult the owner's manual to learn if your GMC pickup is fitted with a brake controller harness. If so, locate the harness -- usually taped to the steering column or to the main wiring harness running up the left-hand side of the driver's footwell -- and press the terminal multi-block on the end of the wires from the brake controller into the terminal multi-block on the end of the harness.
Connect the four wires from the electronic brake controller separately if there is no preexisting harness: Route the red wire to the brake pedal microswitch and connect it with a cable splice to the switched wire.
Connect the black wire to a 12-volt power source that is always hot when the GMC pickup is running. This can be the positive pole of the battery or one of the two studs marked "AUX" on the fuse/relay board located adjacent to the driver's-side wheel well in the engine compartment. These studs are typically protected by 30-amp fuses; consult the brake controller's instructions to learn if this is adequate or if the fuse rating must be changed.
Route the blue wire back through or under the GMC pickup to the hitch. Remove the hitch cover, slide the blue wire in through the small hole in the case where the other wires enter it, then connect the blue wire to terminal two and replace the case. Use cable ties to secure the wire where it is not susceptible to accidental damage.
Use one of the many screws already driven into the metal panels below the dash to connect the white wire to ground. Use a crimping tool to connect an eyelet connector to the end of the wire, use a screwdriver to extract a screw and slip the eyelet connector over its threads, then reinstate the screw.
