Heater and A/C controls not working.

I have a 97 Ford Ranger. 100500 miles 4.0 liter. The problem I am having is that out of the three controls on the dash this is how they work. The fan knob works fine, the temperature knob has no control. All I get is hot air no matter what. The contol switch for a/c heat etc. It will trun off the over all a/c and heat but all I ever get is defrost. So this is how it works Defrost with high heat blowing or nothing at all
I have taken out the panel and replaced it with no results. It still does the same thing. I am told that this may be a vaccum problem. Where do I start if this is so.