Hmm, don't know how a thermostat would make hot air blow when the A/C is on. There are 3 possibilities I can think of, 1) your a/c is not working, possibly a leak or low pressure and you are just blowing outside warm air in, 2) one of the many regulator valves that controls the flow of heat and a/c went bad, 3) your hvac dash controls went bad. The following document (large file) may help you understand how it all works and see if you can fix it yourself.
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