The heater requires that there is hot coolant running through the heater core.
I would first check to make sure your car has coolant in it.
If it does, you most likely have a stuck closed thermostat not allowing coolant flow. It is a relatively common problem on these cars.
Fortunately the thermostat is cheap and easy to replace.
Here is how:
I hope this helps! Let me know if you have any more questions!
But if it was a thermostat/coolant problem, wouldn't that mean that I would have no heat at all? Rather than just when the car slows down? Again, my heater works great while the car is moving, but goes from warm to cold air when the car goes from drive to idle, and the defroster doesn't work at all when I first turn the car on. Could it have something to do with the blower fan or something like that?
I don't mean to sound like I'm second-guessing you, but I just don't want to be expecting a simple, inexpensive fix and then get socked with a huge bill...which has been known to happen to me or more than one occasion!
I understand your concern.
It is most likely a cooling system problem, whether it is a circulation problem, low on coolant or something like that.
It would be best to start be looking into the most likely and cheapest things, before attempting to look into other possibilities.
Let me know what you find!