Check to see if the seals are pulled tight in spots. Over time, they will shrink and pull away from the "sharper" bends of our door frames.
If so, you will have to peel them up and re-apply, leaving a gap in the bottom of the door frame. This gap can be filled with a donor piece from the junk yard. (See link Boosty supplied above)
Most likely, water is leaking down the headliner from un-plugged drain tubes and going for a ride along the seals and settling in there.
The fact that you have water behind the front seat, on the floor pan makes me think the rear drain tubes are unplugged.
You need to get in there and see what is happening.
Please let us know of your findings.
2006 Pontiac G6, 2.4L Ecotec
1993 SSEi "Bonnesaurus" sold regretfully 10-27-10