To do it properly:

Remove rear parcel shelf speakers
Remove rear seat side bolsters
Remove rear seat latch covers and remove parcel shelf
Unscrew bottom of inner C-trim pillar covers and unclip

Access is then available for the 3 x 10mm plastic nuts.

Remember though to put a small piece of tape over the outside covers to hold them on or they will fall off when you undo the last nut and most likely scratch your car on the way down.