+1 It's probably your heater core. That thing is kinda like a little radiator. It probably has a hole in it, the lines are probably corroded especially if you let water sit in the car instead of radiator fluid. It could also have a clog somewhere in the system which causes it to leak in your vehicle.

If you try running your A/C or maybe turn it on to defrost the windows it might cause this leak also. Look under the car around the firewall on the passenger side for a small tube hanging down. This will be the drain for condensation on the AC. Carefully poke a wire up this hose to clear the blockage. Remove the trim piece which is just below the wipers. See if the cowl where the a/c & heater inlet is. Check to see if it is sitting properly or if its cock eyed or something. It might not be seating correctly or it might be corroded, rusted and causing it to leak from there.