Water leaks are not uncommon with our cars. The first thing you need to do is get the water out and dry thing up. This will help prevent rust as well as mold and mildew. Don't drill any holes, use a wet/dry vacuum and towels. I would recommend also pulling up the carpet...you probably have water on the floor board.
The rear wheel wells and undercarriage of the car needs to be looked at for holes and rust. Then check the rear door seals to make certain it'* sealing.
If you have a sun roof, the drain tubes can come undone and or get plugged.