I read somewhere that you have to carefully do this (piece is VERY fragile) with a small screw driver and basically push it away from the window. Don't know if it works but what I read.
Use a thin object on both sides nearest to you and pop up the cover. Then slide the cover to you and it will release . The bulb is then poped up and replaced
If you open the trunk and go under the 3rd stop light, you should be able to pull the lining of the trunk down so you can change it...
The third brake light on a 2007 Toyota Avalon is replaced by first removing the rear, retaining screws. The wiring harness can then be disconnect and the light bulb removed.
Above the brake pedal, on top of the pedal lever.
You take your foot off the brake pedal.
change the fuse if not replace flasher
brake light switch above the brake pedal
what do you have to do to change the center brake light
Step by step procedure for replacing the high brake light, with pictures. https://picasaweb.google.com/thaalsm/ToyotaAvalonHighBrakeLightReplacement
On my (2000) Avalon the centre brake light is part of the middle speaker on the rear shelf. The speaker slides straight forward (you have to tug on it to overcome the friction mounts that hold it in place). Then lift it up and you gain access to the brake lightbulb.
you need to change the multifunctional switchthis happened to mine to
In the top of the trunk near where the brake light is located, there is a small plastic window, remove it and you access the light bulb.