Hi,
To get to the top brake light you need to prise out most of the plastic trim to release it.
Start with the big main panel on the boot. Just take out the little round covers and unscrew the screws with a torx tool. The panel should then come off with a pull.
Then go up each side of the window. There shouldn't be any screws so you would just pull the panels off. The panels are held in by plastic self-gripping bolts. The only way to get them out is by tugging. There shouldn't be any damage caused but it does feel as though something will snap when you are pulling.
The brake light cover is then easy to get to and there should be just two torx screws holding it in.
(This might be a bit late for you as you asked a few weeks ago but I hope it helps)