It would seem that your air mix actuator is either broken or out of calibration. Pulling the glove box and looking through the hold in teh support to the left should set your eyes on the actuator. It'* a little white arm with a metal rod attached. When you operate the controls it should follow soon after.
If you see it trying to jerkily move or clicking, then the actuator would be bad. A dealer will remove the dash to change it out, however it is possible to contortion your arm and do it w/o pulling the dash.
00 Regal GS: All kinds of cool stuff.