Heater Question

[eightbanger]

When I bought my truck the heater would divert air to the windshield when deice was selected and to the foot well when needed there, but now it will only blow air through the diverter balls and the center vent, this happened only recently, the A/C does'nt figure in the problem as all the vacum stuff and hoses had been removed before I bought the truck. I have taken all the heater control stuff out and checked all the various cables and diverting doors inside the heater boxes under the dash and everything is opening and closing fine when I slide the selector knobs on the control panel. I can't figure it out, did I hear somewere that these trucks have a heater thermostat in them that doe's something when the cab gets to a certain temp?? if so maybe it's broke.