Have you ever seen any Dart, Valiant, Demon or Duster with a rear window defroster?
My 1965 Barracuda has a blower motor in the back to blow air on that HUGE glass. No heat, just air. The under dash switch was missing when I rebought the car in 1986 but read "Rear Blower" if I remember.
My mom had the dealer install a heater core from a mail order place like JC Whitney and they ran the heater hoses under the car with a cable controlled shut off valve. It all was missing when I got the car back.