On our 1989 240. One morning our battery was dead and we traced it to the central locking. Apparently shaking the driver's side door would make the relay click so I assumed there was a bad connection or wire inside the door. I took apart the door and found that one of the wires had bare copper showing and when it touched grounded metal the relay would click. The location was directly below the lock rod. From years of locking and unlocking the doors, the wire rubbed against metal finally causing it to lose its insulation and expose the wire. Fixed it by wrapping in layers of electrical tape as well as the other wires that were next to it. Adding shrink tubing would probably help even more if added but I don't have any. Hope this helps someone.

If you need to drive the car in the meantime... just remove the central locking fuse located behind the panel near the driver's left leg and the battery won't get drained.