I am having what seems to be a similar issue on my 1987 190D, on both front door locks; you can put the key in the lock and turn it, and even wiggle the lock knob thing on the inside a bit, but not enough that they door will actually unlock. Which is a problem, as it means that I can't lock my car.

I only bought this car about a month ago, so I don't know how/when this started happening. Apparently in my car, there is a vacuum pump in the back, under the backseat, which controls the doorlocks, so I'm going to take a look at that and possibly replace it with one I pulled from a junkyard 190E (which has donated several other parts to my car, including a tie rod, horn, fusebox cover, lugbolts, transmission fluid dipstick, and heater pump).

Hopefully that will work; otherwise I'll probably be pulling off door panels and wiggling around digital cameras at odd angles as well!