Please help me with AC vents I am confused

[MVK]

I did a search for the vents on this forum and too many replies too many suggestions. I am confused. Please can some one help me. I am not too keen on removing the dash board.

OK this is my problem on 1985 300D turbo, 4 door sedan 123, 179,000 miles:
When I press the defrost button all air comes out thru the defrost vent on top of the dash board. Seems like this is normal.
then
when I press anyother button, things work fine- I mean the flaps for the center and bottom vents close and open properly I checked it with a flash etc. but the defrost air vent on the top of the dash always is on and send air thru it. As a result my center vent throws air at much lower velocity. and there is a lot of moisture condensation on the outside on the windshield due to the cooling caused by inside air from the defrost vents.
Does the defrost vent always stay on( I drove the car for 2 years thinking this was normal- how silly of me.)

Any quick suggestion seems like a very typical problem with a pinpoint solution. I thought may be someone had an exactly same problem and found a fix for it PLEASE HELP
MVK