If you mean water under the rear seat bottom cushion, very slim possibility that its coming from the door weather seal(s) as the water would collect on the floor foot well under the carpet. If the water is laying under the rear seat bottom cushion, then it is most likely coming from the sun roof drain(s) or from a leaking rear window. If the water is coming from the sun roof drain(s), either the drains clogged or the tubes disconnected or cracked, you can test this rather easily. Sit in the car with the doors closed and have someone else spray water from a hose onto the sun roof area only (have them try real hard not to get water running onto the rear window). If the leak is bad enough, you will see the water trickle down along the inside wheel well and start to pool on the area that the rear seat bottom sits. You might also find it starting to come out from around the rear C pillar passenger light(s). If after running water over the sun roof, you don't get any water inside, then have then run water over the top and side of the rear window and look for water again. This should narrow down where the water is coming in at, then you need to fix it.