200 Blazer Heater issue

[1966gip]

I have a 2000 Blazer (s-10) 4x4, with automatic climate control issue. My A/C worked great all summer long, I bought this blazer in April this year 08 with 51K miles on it, now that its winter and I go to use the heater, here's where my problem starts, the fan works in all speeds, it diverts air to vents defrost, and feet no problem, the issue lays in that I can only get heat by rotating the selector switch all the way over to the red hash mark. then I get roasted out of the truck, if I move the selector switch back to a lesser setting all I get is cool air coming from the vents I selected, any vent I select, I have check vacuum lines till I am blue in the face, I seen on here some talk about a thermocouple? a blend door? a recalibration procedure? resistor etc.. any help??