1999 E430 Rear Power Door Lock.
 

OK,
In reading the various posts on the power locking system, I gather the power locking system is vacuum operated. But more precisely, can anyone tell me where to start when ONLY the rear driver's side door does not lock or unlock with the rest of the car? Also, the trunk takes 15+ seconds to open when hitting the console button or the key button. When locking the car I hear a hissing sound coming from the rear seat area. The rest of the car locks and unlocks instantly. By the way, the problem door is the same door the window regulator was replaced in last year. Should I be looking for a damaged vacuum hose or what?
THANKS.

I would check for a vacuum leak / disconnected hose first. Look inside the suspect door first, then check out hoses starting at the pump (under the seat next to the battery).

Easy fix.
 

Ok, sorry for the delayed update but I was able to fix it quite easily. I recommend anyone having this type of issue to first check behind the dust boot of the door in question. I found the hard plastic tube broken inside that boot. I then found some refridgerator water supply hose and stuck the broken ends of the other tubes inside it and glued it together. All done and more durable than before!