If you need to remove the rear shelf, rear seat trim, c pillar trim and shelf side trim pieces all need to come out. Then there are some plastic push pins that come out, you can see them when you fold the rear seats down. Then there are few wiring harnesses that need to be unplugged. Once everything is loose/detached, the whole shelf trim piece slides toward the front of the car to release.
May have missed some details, I did this back about 6 months ago to fix a rattling 3rd brake light and reciting from memory. Need to do it again soon to fix more rattles with heavy bass.