I have a 2005 Chevy Aveo. We park it outside. Any time it rains, water accumulates on the floor of the car. When you begin to drive, it pours in from beneath the dashboard onto your feet. Does not matter if you park uphill, downhill, flat, or if you turn the vents to head, head & feet, feet, defrost, or defrost & feet. Had to use the ShopVac to suck out two inches of water from an overnight storm.
I have an '07. It has the same problem. Fortunately, mine still has some warranty left, so I’ll see what the dealer has to say about it next week. When I got the car last year, the floor was damp under all the pads so I left it in the sun with the pads lifted for a few days to dry out to avoid mold growth. Now I know why.
Most likely it is in the area where the hvac located. If your car has a cabin filter it might be leaking down onto/past the filter and into the cabin.
Likely a blocked drain for the area under the vents right in front of the windscreen. The water goes in and has to come out somewhere. Cleaning it out will also likely eliminate that moldy smell you get from time to time.
String trimmer line may unblock it.
Also consider it may be a drain from the sunroof blocked, if you have a sun roof.