Mike,
The other guys are a lot more knowledgeable than I am about this, but if you get cold air through the defroster, then you must have a water flow problem either through the heater core or getting to it. I just looked at my 69 Cobras heater hoses and the input hose comes out of the intake just before the thermostat and the output hose from the core goes into the water pump. If you can feel your hoses, when the car is good and warm, you should be able to feel which hose is hotter. Even with the heater turned off, it will still have water in it. If one is hot and the other is cold, then it almost has to be a problem in your core or the switch that lets the water through. I have had to replace mine once.
Ron
