2000 Pontiac Grand Prix Weak Air Flow in Dash

There are vent doors in the HVAC system system that direct the air to the location you want. One of them isn't working properly or is stuck. The only thing you can do is start checking. If I recall, this vehicle's vents are actuated by engine vacuum. Check to see if there are any leaks. Under the dash or on the engine. If you are lucky, you may hear a hissing or whistling sound from the leak when the engine is running.