Does the GP have a connector right by the door for the harness or is it wired directly to the interior of the car? I know the Regals have connectors right inside the body of the car and those pins might be corroding etc, may need to be reseated as they can push out when you reconnect.

Also the wiring inside the rubber boot gets bent continually and can/will break and cause shorts. Had a Regal with a totally dead rear driver door. 5 of 7 wires were broken.