Crusty check the wiring loom on the left hinge for breaks - this is an old problem with E34s. Remove the plastic covering and you my find one or more wires split or cut through completely. It happens over time. If you find nothing wrong here then pull the back seat and check the wiring loom on the right hand side (NZ driver's side) where it goes through a hole into the boot -sorry, trunk. This is where mine was damaged.
Phil