I did this a couple of summers ago on my '06. I wasn't difficult. I'll look around today for the article or document I used as my guide. It may take till tomorrow, sorry.
Not hard. Top up, boot open, unlatch and raise the rear of the top and prop up with the latches. You'll see the screws that hold the trim pieces in place. Remove the screws and the trim pulls up and off. The third brake light is slipped between the two pieces of trim. Slide in the new trim and replace the screws. Done.
