Check the wiring in the door jamb that is inside the corrugated boot. You can slide the boot through the hole in the body to expose the wiring. You will probably find broken wires. The drivers door opens and closes more often the other side so this will usually happen there first. The insulation gets hard and brittle and cracks. Then the wire bends at the crack with every door movement, the copper fatigues and breaks. A butt connector will fix it.