Does the air ever come from any vents other than the defroster and floor vents on any setting? The defroster at least (I'm not sure about floor) is the default setting for the doors as a safety feature. If you are unable to get air to come from any other vent regardless of setting, I'd suspect you have no vacuum going to the HVAC controls. It's pretty common for the hard plastic vacuum lines to get brittle and break. Bridging the broken area with a length of rubber vacuum line is a pretty common fix.
-Rod