To fix this, in my classic "I have nothing to lose" fashion, I started twisting the AC/Heater knob back and forth rapidly for a few seconds. I did this until the heater worked. Often, I'd turn off the heater and fan when I left my car and found out that when I turned it on again the heater didn't work. So now, I just don't turn the heater off and everything is fine. If it's too hot in the car I roll down the windows. I'm pretty sure it's just a small, easily fixable problem with the mechanism that switches it back and forth from hot to cold and not anything too serious or expensive. I recently bought a Haynes instruction manual for it online, so when it arrives I'll pull it apart and fiddle with it. If I come up with a better solution I'll post it.