96 bonneville Water leaks to back seats'floor

Rain water leakes to the back seats’s floor. What can be done?

Water leaks in. While vehicle is in motion or does that not matter?

If it leaks while driving in rain, the water may get in under the windshield weather stripping and run towards the rear of the vehicle and drop down through a channel and onto the floor.

Don’t forget water (like air) will take the path of least resistance.

Is there a sun-roof in this vehicle?

Check the weather stripping around the door (if 4 door).

Has this vehicle ever been in a collision? (like a high speed impact) I’m thinking a possible weld crack.

Hole in floor under carpet? Rusted through? Check the floor pans by poking with a screwdriver.