The heater ducts on a 1993 Buick Regal are not working ~ they are stuck on defrost. I suspect a vacuum leak is causing the issue ~ where is the vacuum hose located so I can verify this?
If i remember right there is a vacuum solenoid box under the dash on the passenger side, they are notorious for the hoses and connectors inside the box to disintegrate, you can open the box and re plumb it. You can follow the vacuum lines as all the actuator lines go into it. Be sure to take a picture of the inside before you start repairs.
First check the vacuum supply line under the hood is intact, it will go from the intake manifold thru the firewall. Small diameter plastic line, usually black.