As the other poster asked ( about a new radio being installed ) has
anybody recently worked on the car, especially under the dashboard ?
Have the engine running, then.......
Poke your coconut under the dash & listen for a SSSSSSSssssssss sound.
You may have to remove some of the black lower dash trim panels or
posiblly the glove box insert. Any car I had that had a blown diaphram,
you'd hear SSSSSSssssssss.
Try sliding the control knob back and forth to vent, heat and defrost
and listen for a vacuum leak or any other noises.
Sounds like one of those vacuum diaphram / controls is not operating
correctly so the blend door that directs the heat to the floor and
defroster isn't moving.
Could also be a loose or disconnected hose.
1991 Pontiac Bonneville LE, 3800 V6
_~_~_~_~275,068 miles_~_~_ ~_~_