Could be one of the following air mix/blend/mode door problems, vacuum/actuator and HVAC control module.
December, 8, 2009 AT 8:04 PM
Had same problem with 2000 GMC Safari. Don't know your situation, but mine was missing a vacuum hose. Right now the damper is back to the -default mode- if you will, so your windshield can still be cleared. What I had to do was take off the inside motor cover and all components under dash (not reall that hard just time consuming). The missing vacuum line went into the engine by the drivers side near the middle of engine, off the pcv valve. Now from there it went to the front (under hood) to a tee fitting. Then from there one went down to canister, the other into van. Total cost was near 10 bucks for new hose 5/32 was size I used. Any other Q's and hpefully this helps- Goodluck