Hey everyone, I searched far and wide and could not seem to find a guide for installing the rear speakers in a 2005+ convertible (with a stock system), so I set about figuring it out myself and took pictures along the way. The folks at the stereo install shop I went to initially claimed I'd have to remove the rear seat to do this; the good news is that you don't, and it is actually easier to replace these speakers than the ones in the front door.
Step 1: Lower the roof, turn off the car, and disconnect the negative terminal of the battery.
Step 2: Use a trim stick to pop the cover off the corner. You'll then see this:
Step 3: Gently pop the retaining brackets at the top of the panel off, then roll the front edge off, and finally pop it off the lower panel.
Step 4: At this point, you can actually replace the speaker without removing the panel further:
A couple things to note: 5x7 speakers seem to be a perfect, direct fit. However, in order to get mine to fit, I had to bend a bracket at the bottom of the speaker up:
I purchased a set of wiring harness adapters that clipped to the speakers and allowed me to plug directly into the factory wiring. This seems to be a great (and cheap) solution.
When tightening the screws on the new speakers, remember to gently hand-tighten each screw down, to avoid bending the speaker frame.
Step 5: Put things together using the reverse process. I had to thump on the bottom edge of the panel with my fist in order to get the clips to lock into the lower panel properly. Also, watch out for the seat belts and make sure they are free and clear before snapping the panel back into place.
Additional note: If, for some reason (perhaps working on the convertible motor mechanism) you need to remove the entire panel, there is a retaining clip buried beneath the end of the seat. I found I could remove it by sliding my hand in next to the seat and prying it out.
Replacing it was a little trickier; I still used my hand to pry, but then used a long set of forceps coming from the top down to position the clip into place.
Hopefully this is helpful to anyone here doing the same thing I did!