Central locking problem on a '92 500E
 

I can unlock all door locks with the key from either drivers or passengers door. I can lock all door locks with the key in the passengers door but when locking from the drivers door the rear knobs have trouble going down and often don't lock. Would this be an electrical or vacuum problem?

It's the door lock switch. I own a '92 300E. I had the same problem. Turned out the door lock switch on the driver's door had a bad vacuum leak. However, it's quite a job to replace without a manual and there is a protective cover over the switch. Took me about three hours to replace.