I just solved this problem on a '91 3er and this may work for you too. '91 3er left passenger door had no power window function, all other windows OK, or central locking function, all other doors, trunk, gas filler flap OK. Problem lay in the body to door harness, both central locking wires were broken along with two of four power window wires. I believe the wires were cut by the flexing of the harness from the simple repetitive act of opening and closing the door. Disconnect the battery and splice the wires back together using butt connectors then wrap everything up with electrical tape. HTH and good luck.