W124 Climate control issues.
 

All of the buttons on the climate control unit (with the exception of the off button) do the same thing. Air always flows from the side and defrost vents, no matter what. What would most likely be the problem? Bad pushbutton unit? The footwell doors seem to be held closed with vacuum, because I can push them open with my finger, and they spring right back. So I doubt it's a vacuum leak. Thanks in advance.

Does sound like vacuum supply problem, The defrost flaps are spring loaded open however the center vent & footwells are spring closed. So you need vacuum to close the defrost & open the footwell & center vents.