You have a D2, the coolant circulates through the heater core all the time. You must be sure you have the coolant jug at normal level. Dampers control air flow through heater core, and your regular heater fan has to work, or you just get light air flow through outside air intake. When you use defrost or AC does fan blow hard?
If fan not running, fuses or resistor pack.
If fan is running, suspect control cable or flaps.
Attached pages from everyone's favorite British detective story, the RAVE.