If you have a blanking plate instead of the cool 3rd brake light, get one and here is the how to:
First pop out the blanking plate which has small slots around the edge for a small screwdriver to aid removal.
Behind the panel are 2 screws, and in the centre is a push fit white peg. Remove screws, and from inside remove roof trim panel and squeeze the white peg to release it.
This is the removed panel from a transtop, the red circles show 2 plastic grommets that need removing, may be different on manual roof.
light panel removed, time to clean and wax it while you have access.
Now drill a small hole for the wire as the large hole is for the white peg. Wire up to the light and re-assemble.
Lamp installed , just the screw covers to pop in. Mark63