There are little motors under the air mixing box (tucked at the back by the end of the carpet under the radio cluster) that operate the dampers to bring in the hot /cold air.
Each motor is enclosed in a plastic case, and this case expands to the point the motor does not work. (design flaw) I think the unit you need to replace is closest to the driver side. A little hard to get at but with some patience it can be removed and repaired, or replaced.
Hope this works for you.