David,

You have removed the lower dash panel before and may remove it again. The driver side ventilation outlet is connected to the heater housing by one hose and two flexible adapters. You might have a loose end or something like that...
Once you have the lower dash panel removed, you might as well peek in the heater house outlet with a flashlight while you are changing vent settings. You should see a valve opening and closing.
If all air vents seem to be clogged up, you need to change the interior filter.

greetingz,

__________________
1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)