no heat in my 72 dart...AC car

Does your car have an a/c heater box? I had this same problem & just noticed that the trap door to the heater core was closed & not allowing air to go into it. The vaccume pulls the door shut. I opened the PS lower dash vent to notice it & there is another trap door with a vaccume pod underneith on the front as well & if you push or pull it open you will see the 1 in the vent open & close. This may help