So much for the quick easy fix. The only parts of that circuit are the switch, the receptacle and the harness, and the switch is what actuates the lamp so I would suspect an overly sensitive switch. If the switch seems to function OK, a loose receptacle could be an issue. Open the door and vibrate the unit and see if the light flickers. If it only seems to do it when the door is closed, I would be back at the switch.