Okay, I'll try to give you guys the short version of the story. This problem has existed for 2 years now, and I'm finally determined to take care of it.
About 2 years ago a body shop replaced my driver's side door (1987 560SEL) which was the victim of a hit and run while I was at work. A while before the hit and run, someone (an enemy or a random jerk) put glue in the keyholes of my driver's and passenger side doors. Since then I had been using the trunk to open the doors until I got around to replacing the door handles. Since the car was already in service for the hit and run, I had the shop put in a new driver's side handle/lock. When I got the car back everything looked great (well, mostly), except that my central locking was no longer functioning correctly.
When I use the key in the driver's side door (the passenger side has not been replaced, so no way to test that) everything works as it should. Either all the doors lock (including trunk and gas) or they all unlock. However, when I get into the car and depress the lock knob thing the central locking does nothing. No pump sound, only the lock that I depressed goes down.
Logically, the problem has to be somewhere which would have been removed/replaced with the door, however I simply cannot figure it out. I was under the impression that the central locking is activated when the lock knob goes down and makes/breaks contact with something. I guess it is more complex than that or I wouldn't be having this problem. Any ideas where to start??

Here are some additional details if they will help anyone figure this out...
1) When depressing the passenger side knob all the doors will lock if the ignition is in the "0" position (normal)
2) When depressing the passenger side knob, none of the others will lock if the car is running, however when the ignition is turned off following this action, all the doors lock. (not so normal)
3) If I lock the doors from the passenger side when leaving the vehicle (like I have some stuff in the passsenger seat...), when I return and unlock the driver's door (with the key) none of the others will open (and the knob tries to "suck" itself back in sometimes). If I unlock/lock/unlock in this situation, everything returns to normal.
4) If I unlock the car from the trunk, all doors and the gas flap unlock. If I lock the car from the driver's door after that none of the others will lock. Again I have to lock/unlock/lock.

I think that's it. If anyone needs more info, let me know! I'll get a case of beer for whoever can help me get this fixed. (no, seriously.)