I have a 2005 Ford Five Hundred Limited. About two months ago a wet spot appeared in the middle of my floor mat. It is odorless and colorless, but I only know this from observing the mat. I cannot find the source of the leak, I have yet to see any actual fluid leaking or dripping. It appears right in the center of the floor mat, just before the brake pedal. I dried it up, it reappeared a few weeks later. I took it into the Ford Dealer, and they suspected it was the AC's Heating Core. They pressurized the system and tested it, and found no leak, so they returned my car to me free of charge. Three weeks later, the spot has reappeared. I have a suspicion it still is related to the AC somehow, as when it reappeared I had the AC on very high the whole day before. I also looked under my car the day I noticed the leak and saw no rust spot, but did see where two pieces of the car were welded together some water dripping off of it. Help!!!!

 

My guess: If you've been using your AC hard then it's normal to have water drip out underneath the car. When AC's work they cool the air to below the dew point and condensation occurs; this condensed water drips away. On a hot humid day when you shut off an AC'd car and then come back you'll usually see a small puddle underneath. Normal. This AC generated water drains out of a tube from the evaporator; and if you're in a moist climate mold may grow in the always moist AC evaporator unit (which is where the air cooling/water condensation takes places) and mold could plug the drain tube - meaning condensed water has to find a second exit, so maybe it's dripping out on your carpet (happened to my G'mother's car). The evaporator unit and the drain tube can be cleaned, just takes labor time.

Your symptoms are visual - so photos are good: fold up a kleenex X6 and put it under where the cabin wet spot appears. Then post up a photo of the wet spot to show the color of the interior drip.