Not a real tough job. 96-up Mustangs go together like snap-tite models. Follow the procedure below and you should be OK. Give me a break if I'm a little off, I'm doing this from memory:
Start out by removing the rear seat.
The bottom cushion has 2 white latches about 8 or so inches in from either side of the seat. You should be able to pull the seat up just enough to see them. Use a screwdriver or similar to push the button on the front of the latch in to release the seat. Once both latches are released, the lower cushion just pulls out.
Removing the upper portions of the rear seat requires the removal of the upper pad in front of the rear package tray. Fold the rear upper seats down to reveal the bolts holding the upper pad. Once the pad is removed, you will see the bolts in the top of the rear seat. The seat is bolted at the top and bottom. Remove all of the bolts and pull the upper rear seat assembly out all as one piece.
Once the rear seat is out of the way, concentrate on the upper rear quarter panel trim piece surrounding the quarter window. You must use a torx-50 to remove the shoulder harness mounting point. There is a push pin just below the shoulder harness mounting point that will need to be removed. CAREFULLY pull on the entire panel to release the clips holding it from the inside to the body of the car. These clips are very easy to break. I try and pull on the panel until a locate a clip, then apply even pressure right at the clip to release it. Take your time.
Remove the door scuff plate. Just pull up evenly, it's just clipped.
You're finally to the point of removing the quarter panel.
Remove the small push pins that hold the door seal to the panel at the front.
Remove the one or two push pins that you revealed when you removed the upper quarter panel.
Remove the push pins you revealed when you removed the back seat.
Apply even pressure to the entire panel and pull it towards the front of the car. It should pop out fairly easily.
Repeat the procedure to put it all back together.
Have fun!