Mercedes: climate control, climate control unit, mercedes 190e

Question
I have a mercedes 190e 1988 2.3.  When the fan is on  the side vents and windshield blow hot air, but the middle blows cold ac.  I can't seem to get the any of the vents to work below the dashboard or close off the heat.  I tried changing the climate control unit.  No luck. Not sure what to do now?

Thanks

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.