Would low coolant make your defroster blow cold air?

Generally, no. The heater core usually has a direct line from the water pump, if the coolant was so low as to not circulate through the heater core your engine would overheat. Most likely a clogged heater core. Rust and sediment build up in the coolant system over time and cam clog the narrow channels in a heater core, which looks like a smaller version of your radiator. A reverse flush can sometimes clear the core out, but a severe case may require replacement. Any auto parts store can sell you a flush kit for about $10. I've used the prestone one with no problems, since the core may be really clogged, get the system cleaner bottle as well to ensure you get the most crud out possible. Follow all the directions on the kit, and cleaner, make sure you are using compatible antifreeze when you refill. DO not open any part of the cooling system with a hot engine (more than warm to the touch). Doing that can cause severe burns, blindness, or worse if the antifreeze in the engine starts to boil off violently.