According to most Toyota literature regarding the auto-dimming mirror, it is available, or can be added as an accessory, only on the Avalon models w. sun roof. This is not quite correct, it can be installed on any Avalon that has a switchable front dome lite. i.e., one which has a 3-postion switch controlling the dome lite. I just had a mirror installed on my sun-roofless '07 XL.
If you decide to have one put in, shop the internet for your best deal. They're about $200 from most dealers, but I found one for $120, including shipping. Make sure the wiring kit is included.
The mirror incorporates auto-dimming, a compass and a garage door opener. Kinda neat.
Bob Russell