Why am I not getting any heat in my 1992 Chevrolet Cheyenne?

I have a 1992 Chevrolet Cheyenne I can not get the heat to work in. The motor heats up fine and the thermostat works properly. I have replaced the heater core, all hoses, thermostat, heater control unit, coolant and thermostat housing. My antifreeze is full, Any suggestions on something i may have missed?|||Heater door that is not opening, some are electric, some are vacuum, check the fuses, then look for a Vacuum leak.|||did you bleed the air out of the cooling system? air will keep it from circulating.