1999 Chevy Camaro No hot air blowing.

My car would blow cold air out the heater intermittantly. I could turn the foot, dash and defrost button back and forth and get the heat to blow again. I have since flushed and refilled the radiator, new thermostat and back flushed the heater core. That seemed to work. Now, after about 1,000 mi. No heat at all. I have a new heater valve. Where does it install? Is it the instrument switch? Is the door not opening to let the blower air flow over the heater core? Please help with possible remedy.