head gasket ? ! ? !
having no idea what the car has been through, i would first check the hoses going to the heater core which is real easy. when engine and coolant is hot and with heater on full blast, feel both the in and out hoses from the heater core. you can also squeeze them with your hand and get a feel for the coolant flowing through them. If the out hose from heater core is soft but the in hose is firm then that points to a heater core blockage. And the in hose going to the heater core should be hot, if not then that may also mean heater core blockage but also mean problem upstream back near the thermostat outlet. If both the in and out hoses to the heater core are hot, and it feels like coolant is flowing through them then the problems are either with the hvac unit in the dash, with the temperature mixing door which I think is buried under the dash, or could be with the temperature cable from the hvac unit to the temperature door.
There are also a whole other set of diagnostic steps, but i'm assuming everything else works such as when it's on vent only you get air out of only the vents, a lot of the hvac functions rely on vacuum from the engine and vacuum hoses, but if it's strictly a no hot air issue then i think you can rule the vacuum side of everything out because temperature is controlled by a cable from the temp **** to the mixing door. Worst case and this happened on an old ford truck i had, the mixing door is all plastic and it's a creased plastic hinge, after so many operations the plastic door hinge just breaks and you get cold air all the time, which means replacing that whole mixing box.
oh, and the other thing is make sure the cooling system is completely full and that the overflow reservoir is full on its dipstick, otherwise you may have air in the system which is all collected in the heater core which will cause no hot air. check this first, then check to see if the heater core is blocked.