No heater?

The part he is talking about is called a blender door. By moving the switch from cold to hot will allow the blender door to function. It is either vacuum driven or cable. In some cases electrical. That coud be a possibility. You should also check your heater hoses to make sure both are hot after the thermostat opens. If one heater hose is cooler then the other then you need to look at the heater core for being clogged up. If so, by changing the heater core should solve your problem. If that's not the case then look to see what kind of switch you have, ie. Vacuum, cable, or electical to find out what other diagnosis needs to take place. Also the knob could be cracked on the inside, so when you turn the knob it's not moving the switch to allow the blender door to function. Let me know