I just finished this repair today. First, the problem is a switch that is soldered into a circuit board on the back of the A/C Controller Assembly. There is no practical way to repair this switch nor swap out any sub-assembly. The entire controller (which consists of the 3 rotary knobs and the 2 push-button switches) must be replaced. The switch fails in the same way a single button will quit working on a TV remote control. I purchased a used controller from a junkyard. The online parts manual shows that the original controller has been superseded by a new part number. I also noticed that my old one did not have any orange illustrations on the mode selector, but the newer one I purchased has matching orange illustrations for the fresh air knob and the defrost settings.
To swap out the controller involves taking off the top "Instrument Finisher E", the "Cluster Lid C", the "Audio Unit", the "Instrument Finisher D", then remove the "Intake Door Knob", followed by the "Controller Finisher", and finally the "Controller".
The hardest thing to figure out is the 3 control cables, and how to release them. Each cable housing goes into a "socket affair" that has 2 plastic spring-loaded "grippers". These have to be spread apart to allow the cable housings to be extracted. I had success using 2 different types of snap-ring pliers. They both expand a snap-ring when the grips are squeezed, and I needed both straight and right-angle pliers. The cable housings must be inserted completely on re-assembly, or the grippers will not do their job and the cable housing will slide back and forth. It may not be abundantly clear, but the ends of the plastic cable housings are "molded" and will only properly seat if the housings are rotated to 1 of 2 positions (there are flats on 2 sides that forms a bump that is gripped).
The other thing that gave me trouble was the big black electrical connector on the controller. The tab that must be pushed in to release the connector is on top, and almost impossible to depress with fat, stubby fingers. Persistence will get you through.
2 final thoughts: make sure you disconnect the battery cables (negative first please) or you will have an airbag error code when you are done (you have to disconnect the airbag warning light harness). Clearing this code is a PITA and involves considerable research to find the procedure. The second thought is, this procedure should not be performed around children, mothers, wives, and especially "church friends". Failure to heed this warning could result in the diminished perception of your character.