I would check the door switch and/or the wiring to it. My son had this problem and the wiring connections to the door switch had got hot and blackened the wires. I cut the terminals off, cut back to good copper wiring, fitted new spade terminals and connected to the door switch again. Bob's your uncle!
Of course it could just need a new door switch itself if that has an internal fault.
ChicksFan, April 2015
