Try the :civinfo: wiki, specifically "Frequently Asked Questions" section...
"Puddle of water under the car
This comes up all the time! The aircon has 2 functions - to cool the air and to remove the water. The water removal is why you should leave the aircon on in the winter, to stop the inside of the windows from misting up.
The water that is removed from the air is discharged through a little pipe to the underside of the car. Sometimes when you park the car, a small puddle of water is left from this tube on the ground, and is quite normal."