97 Hombre climate control oddity

In short, the climate control is stuck on defrost.
There are three knobs: the first one sets the temp from hot to cool and works fine.
The second sets the fan speed and seems to work fine.
The last one changes between defroster, A/C, vents, floor vents, and so forth. When it put it on anything but off the fan will come on, but it wont change where the air comes from or switch on the A/C condensor.
The switch doesnt feel broken and it does activate the fan when its switched to any setting, so I dont think its the actual switch.
So what am I looking at replacing? Can you give me an idea of what parts (GM maybe?) are compatable? Thanks!
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