No heater?

I have a Ford Explorer 2002 who's heat decided to give out last week. Coolant and thermostat replaced, still not working. There is a knocking noise and cold air does flow with no sign of leakage from coolant. Mechanic explained the "unit" has like a door that needs to be opened and is stuck in the closed position, he unfortunately is unsure of the name of item and insists there are no parts and pieces to buy only to replace the "unit." Winter is fast approaching any help would be appreciated.
Thanks in advance

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

Thanks a lot, it was the blend door actuator, it cost me a little over $400. but I wasnt even willing to try digging around back there, so thanks so much!!