Before yanking out the main harness (from the engine compartment through the firewall to the voltage regulator under the back seat), cut off all of the extra wires on the old loom (tail lights, license plate light, etc.) and lash the new main harness tightly to the old one with electrical tape (make sure no loose wires are sticking out that can get snagged on the firewall). Get a friend to help pull the old harness out while you feed the new one through the hole. It is important not to lose the connection through this conduit.