The driver door actuator should be the first one to inspect after you have determined your pump is OK. I think beginning 1985 Mercedes made a change with the vacuum pump to assist the operation of the lumbar feature for the driver seat. If your 190 has this feature, there should be a dial similar to the speaker fader switch on the right side of the seat connected to a vacuum or electric connection line. Under the seat are vacuum lines to inspect as well.
But first, remove the rear seat bottom to inspect your vacuum pump. Look for vacuum lines that are crimped or unplugged. Unplug the vacuum line from the pump. If you can make the pump work while you are inspecting it, plug the vacuum port with your finger after the pump begins to work. This should cause the pressure switch inside the pump to shut it off in a couple of seconds or so. If not the pump is bad. If the pump seems to shut off quickly, begin following the vacuum lines. Remove the passenger carpet. There are several vacuum line junctions here. Inspect for damage or unplugged lines. Open the trunk and inspect the vacuum actuator for the fuel door. Test it's operation manually if you can. Best to remove it to inspect.
Unplug the vacuum line to it, plug the vacuum port on it and see if you can move the plunger in and out with the port plugged. Yes? It is bad. No? It is good. The trunk lock, and each door has a similar actuator. If the problem is not with the pump or in the trunk, best to begin with the driver door actuator. Don't forget to inspect the vacuum lines as they pass from the doors to the car. Could be a pinch in these areas too.

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