Fan speed switch

Your fan speed resistor is bad and needs to be replaced. It is usually somewhere around the blower motor and screwed to it. Sorry, I don't know the exact location on your car.

You will need a blower motor resistor. I had the exact same problem altho I don't use my AC. But regardless the motor blows either hot air (when using your heater) and cold air (when running AC). The speed of the motor is determined by how much voltage gets to the blower motor. The blower motor resistor regulates that amount of voltage based on how much you turn the knob on the front dash panel. If the resistor is bad then the voltage is either 100% all the way with knob on highest setting or 0% because knob is shut to off position. I promise u this is the problem. I had exact same thing. Only worked on highest setting or none at all. It's an easy fix essentially BUT it is a PAIN in the A** to get up under the passenger side dash. Remove the glove box door, don't take off entire dash tho. look for the roundish blower motor. Should take that off. Take off blower fan HOUSING, inside u will see the fan which literally looks like a hamster running wheel. Literally. Take off. U will see old blower motor resistor in between where that wheel you took off was and the solid front firewall panel of the car. You can get it off but a total pain because not too much room to work. U will need small sockets, ratchets, screwdrivers, etc to get it off. Very small bolts and u will be laying up side down with your head literally under the dash board area. Take ur time but u can do it. Took me about 1 hour and a few swear words lol but worth it. All speed settings good and also saved about $225 labor ( in my area ). Good luck bro. Hope this helps

Forgot to say my car is 2001 Pontiac Grand Prix GT so probably identical to your set up man.