If not A/C condensation, then it has to be either coolant or rain water. Take a close look at the liquid to determine exactly what it is.

If it was raining were you running the defrost as this uses the A/C system and would create a lot of
condensation.

If it is cooland you may have a leak in the heater core or connecting hoses......more likely the core though.

Or it most likely could be the following.....

http://www.nichols.nu/tip757.htm