Good eye, the TD's and D's rely on a vacuum powered shutoff. Best thing to do is go through the visible plastic vac lines and rubber connections and look for leak points. Replace cracked rubber connections with new MB hoses or Y connectors. Electrial tape can serve as a quick fix until getting parts.

Sometimes it will be in the motor compartment, others in the door locks. If you do a fairly thorough search and can't find it, your dealer / service shop can do a check for where the problem is.

Its a little embarrasing opening the hood to shut off a car but we all have done it time to time!

Hope this helps,
Steve S.