I searched and didn't see anyone mention my exact experience, so hopefully this will help someone in the future
I bought a 2012 about 3 months ago, and the previous owner mentioned that the passenger floorboard flooded once and the dealer "fixed" it. We just had torrential rains in northern AL and i vacuumed at least 3 gallons (not an exaggeration) of water out of the passenger rear and front floorboards.
It was way too much water for an a/c drain or antenna leak, so i figured out it was coming from the fresh air intake area at the base of the windshield by tracing the wet jute padding. This intake is under the plastic cover at the base of the windshield that the wiper shafts come out of.
So, i removed the passenger side wiper arm, took out all of the plastic fasteners, lifted it up and looked in from the passenger side. You'll see a large air intake in what is essentially a tub (great for holding water evidently), and a rubber drain hose at the bottom. My hose was stopped up (tested by pouring in water), and the blockage was near the bottom. I guess the PO parked under a tree with small leaves or the like for a couple of years.
Anyway, i'm still trying to dry the carpet out but at least it won't do it again.