A/C/Heat/Defroster not working, ideas?

I have a weird problem, if I press the AC, Heat, or defroster button, the same thing happens. The A/C clicks on. No matter what button I press that's all I get, no defroster, no heat, regardless of where I set the thermostat.

Apparently, something has gone screwy. I thought it might be a relay, but I didn't see a relay specifically for the system. Did I miss it? If not a relay, any ideas?