Mercedes: 300 TD Mercedes-Stuck on Defrost, vacuum problem, upper flow

Question
I have just purchased a 1992 300 TD 2.5 and had the compressor for the HVAC replaced.  Once replaced I found that the air blows out the side vents and the defrost regardless of switch position.  I can turn the unit off/on by pushing the 0 and any of the switches so the switches seem to work fine.  Is there a valve that turns the air flow in the different directions?  I noted in the manual that the side and upper flow is indicative of defrost mode.  Are there other suggestions?

Answer
best guess:
This sounds like a vacuum problem, these can take months to trace/repair as a DIY weekend project.
Where are you?
I may be able to suggest a good local MB shop for diagnosis/repair.
With a little luck, they will be able to fix it under one hour.