Check careful using your nose (or someone else's if you know someone with a good sniffer) and see if it is coolant or just water. If it is coolant, it is the heater core, if it is just water, it probably is the A/C condensation pan is draining insdie the car. This could be caused my the drain line clogged, cracked, or broken. You might as well open it up and do both at the same time. The parts probably aren't that much but it takes a little figuring out how to get to it. I knew one guy who about flipped out one time trying to change a heater core on a VW Jetta. Lotta yelling and bad language.
Last possiblity that I can think of is a windshield leak. Has it ever been replaced?