Do you smell coolant through the vents from time to time? Have you checked to see how quickly you are loosing coolant?

You can leak coolant where the hoses come through the fire wall, or you can leak coolant where they connect to the heater core. These leaks would just drip onto the carpet and not likely be smelled through the vents.

The most common leak is the heater core. Check the price on Eeuroparts.com I think they are around $100 and you can do it yourself.

Try running the AC for about 10 minutes at the coldest setting and then turn it to the warmest setting and see if any coolant odor comes out - that will definatley point to the heater core. At the coldest setting, the heater core is closed off and the vapor will accumulate and then excape when you turn on the warmest setting and expose the core to the circulating air.