Most probably the rear window guide in the door is broken. Take the door panel off (good videos can be found on how to do that). Tgen check for loose parts inside the door. Replace if necessary.

There should be a torx or phillips screw about halfway down the back edge of the door below the latch. If it is there, remove it and you will probably find the bracket that should hold the window rear guide has broken and falls into the bottom of the door. Later cars used a rivet to hold it in place...

The rear seat is just clipped to the floor. One of the wires that forms the seat frame is slipped into a catch on the floor and at the front edge of the seat. There are two, on on each side. Push the front of the rear seat toward the rear of the car and lift up. You might need to push pretty hard to...