If all flaps areworking fine under diff modes-- meaning defrost, front vent, leg room flpas etc. then its not the problem with any of the vacuum elements or the five change over switches.
Its then the monovalve. Its located on the fire wall in the engine comparment. Easy to test its function and repair is about 30-35 bucks kit and 30 minutes of time. Check the tripod member pages
Good luck.

Let us know what you find.
MVK

__________________
One penny saved is three penny earned.

1985 300D Turbo 179,000miles