To replace a GM wiring harness, photograph the existing wiring harness with detailed pictures of the wire connections. Disconnect and remove the battery from the vehicle. Remove the old harness from all individual connections to the engine. Remove all retainers and clips that secure the old harness in place, and unbolt the starter wiring and alternator.
Jack up the car and use jack stands to enhance its stability. Remove all wiring connections under the engine. Isolate and separate harnesses that intertwine with the engine harness. Based on the photographic documentation and labels on the old harness, label all wiring connections on the new harness.
To install the new harness, plug the connectors into the correct receptacles. Confirm that the shapes of the connectors are specific to the harness being used. Use the code to match the shape of the connections. Use the rubber boot at the end or both ends of the harness for guidance. Ensure that the new harness is well secured and is reinstalled to the position of the original harness.
Tuck all wires back into the respective conduits. Lower the car and reinstall the battery. Test the electrical components after the harness has been installed. Start the car and check whether components such as taillights, blinkers and other parts that depend on the electrical system are functioning correctly.