I just fixed the same problem on my 96 park ave. You probably have a broken air mix actuator. There are 2 on the car but the one on the passenger side seems to fail most often. Open the glove box all the way down. You need to fold in the rubber stoppers attached to the back side of the glove box to release it all the way down. Once you do that you will see a silver metal arm that connects the actuator to the vent door on that side of the car. If you can move it back and forth, you have a broken gear inside the actuator. Local parts stores by me don't carry it so I bought mine through GM Parts Direct for $50 plus shipping.