I just purchased 78 and the air blows to the floor no matter where you put the control. Does anyone know is it vacuum or mechanical control? I have not have had time to dig in yet, what can I look for?  

If I rememebr correctly, the C3 uses a cable to control the vents, or a combo of cable and vacuum. best way to find out is to pull the dash apart (you're going to have to anyway to fix the problem) and if you've got vacuum lines hooked to the controller, or a cable, then your question's answered.

It could also be a problem with the felt seals on the vent doors, if they're gone then the vents won't work right.  

Did you ever figure this out? I am having similar issues...