I have owned an 82 300d for 15 years. the acucator behind the glove box controls the center vents. Its purpost is to divert the air from the center vents to the defroster. Except when on defrost, you should get air from the center vents. Remove the Glove box liner. You will spot the accuator. With the car running, press defrost and then the buttons for the A/C If it is working, you should see the accucator move. It should move full range. If not check the accucator with a vacume pump.
Good luck,
Hal
82 300d 193k