I happened to be thumbing through my owner's manual and wouldn't you know it, it also explained how to do it. I thought I'd document my removal/repair efforts.
1. Pry up on the back of housing. For doing this kind of task I prefer a wide metal paint scraper - thin enough to get between the seams and wide enough not to crack or bulge the piece that you are prying on. You can clearly see the fabric here coming away from the speaker grille.
2. Detach the third brake light by rotating the plug in the brake light housing and pullit down. Note: if you plan on putting an LED in for the third brake light (like I was) think again - the light housing is designed such that the bulb points straight up from the socket, and since in most LED's the light points straight away from the socket end, it wouldn't be much good for this application. I haven't used any of the LED's that have lights all the way around them - that may work.
3. The grille is held onto the housing with four screws.
4. Tack the cloth back into place. I used just a light misting of 3M Super 77 spray adhesive - spray on, wait a few sconds for it to get tacky and then press and hold into place for a few seconds.
5. The finished product back together - ready to go in the car.