Noticed this during a recent trip halfway cross country. It did POUR for part of the trip, but I didn’t notice the wetness until a few days after we were at our destination. Further, none of our luggage etc was wet, so I don’t suspect a leak in the rubber gaskets along the window. We did have the A/C running constantly while in 90° heat. I checked underneath the car; saw no obvious signs of rust on the underside. I did see a hose and some type of contraption on the passenger side rear, above the muffler - it was dripping water. Is this part of the A/C system, and could it be the cause of dampness in the carpet?
I would check real carefully underneath and in the back by the bumper and tail lights for any rubber plugs or grommets that might be missing or not in place. I had a leak on my Pontiac that would wet the wheel well. After much investigation I finally found a grommet out of place by the bumper from when I had a hitch installed. Also start with the hose and flood the area from top to bottom trying to see where the leak is.