Had the same exact problem with my sister's 1984 300CD. Don't take the dash apart. The problem is with one of the vaccum solenoids that opens the center vent.
Remove the a/c push button unit, be careful not to break the wood when you pull it. It is held in place by plastic pins.
Behind the wood you will see some phllips screws, they hold the push button unit in place.
Behing the push button you will see four vaccum solenoids.
3) Center Vent
Not sure if that is the sequence but I am sure it is close. If I remember correctly the one that controls the center vent is no.3 from left to right, to find out for sure:
Turn temp wheel to min.
Turn on car and a/c
Put your hand on the solenoids and try the various functions on the push button. The one that doesn't click should be the one for the center vent. I think they are $29.00 at the dealer.
The button with the arrow down is not to direct air to your feet. This system does not allow that selection separately. Arrow down is normal a/c, arrow up and down is bi-level - hot air only, the others are self explanatory. The arrow down function will direct air to your feet after the system detects it is too cold or when you have a temp selected that requires heat, it will then close the center vent and blow on the sides and feet only. That is the way it is supposed to work anyway. These systems almost never work perfectly.
[This message has been edited by jbrasile (edited 09-13-2000).]