I have a 1996 Buick Le Sabre. There is a bad water leak onto the passenger side back floorboard. I have taken it to the dealer and two repair shops. They are stumped. A hard Florida rain can get 2 inches of water on the floor. Tried reglueing the rubber seal. That did not work. Any ideas?
Have a sunroof? First place to look for problems.
My guess is the windshield.
The seals for the windows that go up and down, those are not perfectly water-tight even when new, and water can easily get inside the door. The door is designed to drain all the water that get in, out. There are drain holes at the bottom of the doors. Over time these holes can get clogged. Make sure they are open and drain freely on all the doors.
Inspect the front and back glass for any signs of rust around the molding. Put the car on the lift and inspect the right rear wheel opening for a rust through spot.
Any front end accidents? My 2000 Blazer had the bumper and left fender replaced before I picked it up. There was a leak in the seam between the inner fender and the firewall. I found the leak shining a flashlight under the dash while running water into the windshield cowl. The flashlight will reflect off any water leaking in.