Does anyone have diagrams that show how the plastic seatbacks are connected to the seats? I have one that is coming loose in the upper corner and have never had the seat apart so I don't know what I'm getting into.

Don't have a picture but here's a description assuming these are original seats. The seat back is actually steel and has two steel 'hooks' spot-welded approximately 6 inches from the top. The seat frame has two upward-facing tabs of metal that fit into the seatback hooks. The seatback is retained by two sheetmetal screws at the bottom. To remove, take out the screws and pull up and away. When you install it, you have to push down and push in at the same time in order to catch the hooks and get the back to fit around the upholstery.
As to the reason for the gap, could be the tabs on the seat have gotten bent or the spot weld on the seat back has given up.

Okay thanks for the help. I'll check it out this week end.