I had the exact same problem with
my '85 300D and found that the connector that connects the "damper" door for the two center vents and the linkage to the vacuum actuator arm was gone. The vacuum actuator was moving but it was not opening/closing the damper door. You should be able to take a bright flashlight and look into the vent. If the damper door is opening you will be able to see it in there in the vertical position. If it is not opening it will not be in the vertical position. I used a tie wrap to connect them back together. The other thing I did (to insure MAXIMUM air flow out of the two center vents) was to tie wrap the defrost vacuum actuator in the closed position. When you mash the a/c button the vacuum actuator for the defrost is supposed to close off and the center vent vacuum actuator is supposed to open. The other thing you will have to check is to see if you have a vacuum leak in there somewhere and insure that the vacuum actuators are working properly. Try to put a vacuum tester on the line going to the center vacuum actuator and put some vacuum on it prior to looking into the vent opening. Pump it up and you should see the damper door open. A vacuum leak or the actuators leaking will keep them from working properly. The vacuum actuator for the defrost is located to the upper right of the gas pedal. You have to remove the kickpanel underneath the steering wheel to get to it.
I would suggest calling the 800 MB number and purchasing the "Electrical Troubleshooting Manual" for the 300D if you do not already have one. It is an excellent manual for use during troubleshooting.