Water in REAR Driver side floorboard

2012 Buick Regal, 4 door, no sunroof, On 3 occasions there has been water under the carpet behind the drivers seat floorboard. Yes, behind the drivers seat. Each time I have taken the back seat out, pulled up the carpet to find 1 inch of water. The water is very clean. Does not coincide with rain that I can think of.

It appears to be wettest up under the drivers seat by the hump. I park on a slight upward grade so naturally it would move to the back if it was coming from further up. I removed the trim and sprayed the outside of the car down suspecting door seals. No leaks were detected. The car does have AC vents under the seats and running through the center console to rear vents.

I know little about cars and the AC, but is it possible the water is coming from the AC? Thoughts?

Completely possible it is from a/c vents-- water could be backing up and not flowing out the condensate drain-- the big hint to this is the answer to this question-- Do you drive for lengthy periods of time at highway speed- say 30 minutes or more, non stop at 60+ mph? If so, the next thing to look at is Do you have a right angle (elbow) tube on your condensate drain pipe? It would be nearly centered on the firewall, engine side in your car. If there is just a plastic pipe protruding out- with no rubber elbow pointing the flow down, then - there-in lies your problem. The rubber elbow creates a chimney effect, that sucks the water out of the evaporator drain pan inside the car- if no elbow, then wind pressure (from driving highway speeds) pushes the flow (or does not allow flow) out the horizontal drain pipe. Once the water backs up high enough, it flows out into the duct work- which in your case leads directly to the back seat vents.

I've seen this in GM B-bodies where the second shift assembly operators did not install this crucial elbow- salesmen who drove this station wagon complained of water on back seat floor only after a 3 hr non-stop highway drive- put the elbow on the drain (or fashion a drain hose) and the problem goes away.. Anyone who knows a/c can easily verify this issue.

This vehicle is driven in town about 5 miles a day. I have looked all over the firewall and see nothing. I will Jack it up this weekend and look again. I appreciate your support.

I got the car up on ramps, took out the airfilter and lines to get a better look at the firewall. Looked from the top and from the bottom. Still can't find the evap drain exiting the firewall. Anyone have a diagram, picture or manual?

Try looking under the front passenger floor pan next to the floor comsole!

It ended up coming unplugged and the condensation was dripping from a hole in the frame on the drivers side.