Water leaking on rear passenger floor

Can anyone help? I have 98 Grand Prix GTP. Major water leaking on floor of right REAR passenger. Noticed after large rainstorm, but i'm not sure this is the cause. Water was profuse and would not dry. It is only on the floor in front of passenger seat, under front seat about midway.
thanks for any help

When the door is shut does it seal tight at the top? I have a 2000 GTP and the drivers side door does not form a good seal at the top when closed and it leaks on my arm in heavy rain. Its been that way since it was new but now I live in Vegas and it never rains here so I dont worry about it. To test it, I would have someone run water over the door with a hose for a while and you get in and see if its coming in at the top where the door meets the roof.

Do you have a sunroof? Sometimes, if it rains hard enough, the sunroof drain tubes can become overwhelmed.........meaning that water is falling on the car faster than the tubes can drain it away.

My friend had had the exact same problem with his VW Cabrio (convertible with leather top). He tried anything to even spray water on the roof and he couldn't replicate it; however, every time it rained at night, he got a puddle of water on front of his passenger-side back seat...

One time he popped the front hood and we noticed that there was a lot of pine needles in the drain channel underneath the wiper. He cleaned out the needles and from then on, on more water.