We had a similar problem with our 91 240. Battery would not hold a charge. We could jump the car, drive it all day with no problem, then overnight the battery would die. Turned out the central locking system was the culprit. I forget which fuse was involved but it was the same fuse as the radio. I found that when removing and replacing that fuse, I got a small arc, indicating to me that there was constant power running through there, even with the car off. I disconnected the central locking system in the drivers door and have not had a starting problem or a dead bettery since. Do have to lock and unlock each door individually now but hey, at least the car starts. You might want to check that circuit.

Good luck!