no heat in my 72 dart...AC car

Has the heat in the car EVER worked?

If not, check the heater hoses. I've seen more than one car that's had the heater core bypassed by hooking the heater hose output on the water pump directly back into the input. This is usually done because either the heater core or water control valve leaks. If the heater hoses are hooked up correctly, then check the water control valve like '74 Sport mentioned.

I would bet on one of these two things. If the heater core is so plugged that NO water circulates (and if any did you'd get warm air), then the lack of heat will be the least of your worries.