All the locks, including the trunk and gas filler door are activated via a vaccum pump system. The pump itself is located under the rear seat cushion on the left/port side. Check the fuse for the pump (8A White #12) in the fuse box and then check the pump and the yellow vaccum lines.
But, before you spend a bunch of time pulling out your back seat and testing vacuum lines...pull out fuse #12, check that it's not blown or corroded, leave it out for about 30 seconds, and then put it back in. The most common cause of your problem is that for some reason the auto-lock/unlock system wigs out every once in awhile, so pulling the fuse out for 30 seconds re-sets the system. I read the specifics about this in a Mercedes service bulletin awhile back but I can't remember what edition.
The same thing happened to me. The doors never locked or unlocked when I turned the key. Try this:
Park the car in a relatively silent place and lock and unlock the driver's door from inside. Listen for the sound of a motor running (This should last about 30 seconds to 2 minutes and turn off by itself). If you hear it, the problem is probably one of the vacuum pipes broken. It could also be that the pump isn't performing at 100% so try to check by connecting another pump (if you have access to one temporarily).
To get at the pump, take off the rear seat (not the backrest, just the horizontal half). To do this, you need to press two red tabs that are just under the front of the rear seat and lift off the seat.
If the pump is connected, it's probably vacuum.
To get at the pipes, you need to start disassembling the interior trim on the doors. Before doing that, do the easy part - Open the trunk. On the right side (behind the fuel tank but inside the trunk, there is a large plastic panel. Remove it and check those pipes first.
If you don't want to get at the vacuum pipes (I didn't), disconnect the power supply to the pump so it doesn't burn itself out pumping like crazy.
Just an added note, there is also a switch on the door lock in the driver's door that activates the pump. Besides having to replace the vacuum pump on a '84 190D I once owned due to a blocked up A/C drain hose that allowed water to get to the pump and fry it, I had to replace the door switch that went bad.
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