1992 Buick Century Back seat sholder strap stuck

There's a trim panel on what's called the C-Pillar. That will, either snap off or have screws that need to be removed. It should come off without removing the seat cushion. If not, then removal of the rear seat is as follows:

There'll be two bolts on the floor, right at the base of the rear seat cushion.

Remove those two bolts, pull up on the seat cushion and remove it. Then the back rest needs to be unbolted at the very bottom in the same way. Once you have those bolts removed, lift straight up on that cushion.

Installation is in reverse. Check first to see if it's even necessary to remove the seat. You may be able to gain access without doing that.