The entire procedure is this:
Remove lower rear seat cushion by pulling up on its corners to release it from the restraints.
Remove upper seat cushion by pulling toward you on the upper left and right hand corners. This part takes a considerable amount of effort compared to the lower seats. If the upper portion has never been removed, there will be an 8mm bolt on each side fastening it in place behind the seatbelt latch. Don't reinstall it, as it serves no purpose.
With both seat sections out, remove the pillar trim on each side. Its fragile, so you'll have to coax it rather than just yank.
Next you need to unclip the rear shelf. Slide a screwdriver underneath the pop rivets, pry them up, and they'll come right out. Now you can slide the rear shelf forward. You can take it out of the car if you wish, as it will give you more room to work, but you don't have to. To remove it completelt unbolt the seatbelt fasteners on the bottom and slide them through the opening.
Now you will have access to the 3rd brake light. It is held in place by 3 screws. Unscrew them and remove the unit.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>