I have a 98 grand prix that the vents does not switch like the suppose when I use my A/C but the perform just fine when using the heat
Sounds like the switch that has all the positions on it may have a vacuum blockage ( the rotary switch has multiple orifices that direct to vacuum to various actuators) Hard to repair but easy to replace ... cheap too ...Check Ebay
Search Pontiac Grand Prix Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Pontiac Grand Prix Experts