Sounds kind of like a Blend-Door issue. Check this thread: http://dodgeforum.com/forum/1st-gen-...orks-fine.html
The way the HVAC works, is that the heater core always has full hot water flow through it. The blend door directs air either through the hot heater core or through the ac evaporator coil, or mixes some of both, to get the temperature you select with the control ****. Sounds like your blend door broke and is probably resting down on top of the heater core, preventing any air from flowing through it (so air just bypasses the core).