If it is water and if it is a small leak and you can't see water coming out under the car when the a/c is on, it could be the the drain being plugged. Big Jim mentioned in a previous post in the ARCHIVES-PROBLEMS that it could be a plugged A/C drain and the drain runs in the firewall. You need to go under the car to find it just above the exhaust. You may want to clean out the undersides first as it is difficult to find the drain. He used a coathanger wire to clear the drain and you shouldn't push in more than an inch for fear of damaging the insides.
On a related previous post, Dell Employee, this is your fix.
