Actually, in this case I believe you are looking for a vacuum leak. The vent doors are actuated by vacuum. If there isn't enough because of a leak, it will cause this. Check under the dash for damaged or disconnected hoses. Also, with the engine running, listen for a hissing sound.
AS far as access, you need to remove part of the dashboard, glovebox. To access the HVAC system and the vent doors.