Thank you Danny and sorry for not being specific. My car is a 350SD from 1991, it is a USA edition chasis #126134. I believe the ACC in the USA version might behave different from the European edition. I thought that at one point when my car was in defrost there was barely any air coming out from the center outlet, even with the external door opened. After sending the car for A/C repair (to change the evaporator) it is blowing cold air constantly through the center outlet even in defrost. I thought they might have removed something, but I cannot identify any part, door or flap missing. The heater/evaporator housing is now exposed, and there is no evidence of a missing door. Steve told me that it should be automatically controlled by the ACC, but I need to get to the door that is missing or inactive. The car was bought here in Puerto Rico through the only Authorize MB Dealer in the Island.
If you happen to know some infromation please reply. I am in the process of getting everything back in place, with new vaccum elements.

Thank you again for replying to this thread.

Darrel