The back corner panel on the drivers side fills up with water and you cannot get it out, weather it rains or you wash the car you will hear it swooshing like a massive wave, except for now it is starting to leak into the car floor boards, just yesterday I shopped vac the car and had 4 1/2 GALLONS on water on my driver side floor boards, front and back.. If anyone knows why this is starting to happen PLEASE contact me, I called the dealership and they never heard of anything thing like it before.
Back Corner- Panel (Drivers Side) fills up with WATER.
There are three (or four?) draining holes in the bottom of the rocker panels on each side of the car. If you don't make sure that these holes are clean, water will collect and slosh in just the way you've described.
This may be covered in the owner's manual (but I'm not totally sure).
Thanks for this comment, I have a 1996 Sebring and the same thing is happening, it sounds like a wave is sloshing around in my trunk, but I cannot find where the water is settling. Now I will be able to take care of it. Thanks.