The rear end of the car is controlled by the PSE pump. Not only does it do the blow/suck thing for the door locks (and seat cushions if you are lucky) but it also controls the rear dome light and rear defroster.

I know there are stories of spilled drinks, leaks, etc that can cause the circuit board in the bottom of the PSE to short out. Or you could have a stuck relay.

The units were used across the 210 range so they are getting easier to find. Unfortunately it requires the HHT or Xentry to personalize ("version code") it to the vehicle.