2004 Chevy Avalanche air conditioning to driver side

Yesterday during a drive home, the air conditioning ducts on the driver side started throwing very hot air, yet the passenger side continued with the cold air. I have taken the cover off at the bottom under the dash that covers things there and can find nothing that could cause it. Where is the actuator that opens/closes the door to each side-can this go out and cause the air not to come out, only heat? How do I get to it