I have a 1999 Silverado blowing only cold air, fan works great, mode works, it changes zones, but no heat and its 8 degrees outside. Any clues? Thanks

 

You may need to get down at the lower vents and verify that the blending door is opening/working. Probably whats happening is the heat is there at the heater core but is not being allowed to enter the vehicle becasue the door is not opening because its stuck or is not working anymore.

 

make sure the rad has coolant in it as well....no antifreeze = no heat

 

Ok, further info: the radiator is full, I haven't checked the door but I did check the heater hoses, one is warm and the other is cold. If the door doesn't open will the water still circulate through the heater core?

 

And then there's the old thermostat ploy too...............

 

If one hose is warm while the other is cold, your heater core is likely plugged....either flush it or replace it

 

I had the heater core flushed and it fixed it. Thanks guys