I had the same problem this year. Like an inch or more of water in the pass side rear floor. Turned out to be a rust hole in the front side of the rear wheel well. Right where the carpet ends in the corner of the floor under the seat bottom. I dug it out with a screwdriver and caulked it really good with roofing tar in a quart can. Used a putty knife to spread it. That stuff is awesome, it has long hairs of fiberglass in it and fills huge holes really well. No more water !