I posted the factory service manual instructions for installing satellite radio here:
Anyone have a tech writeup for pulling the radio out of a 05 dash?
There may be slight differences in the 08 interior, wire loom locations, and what-not, but the procedure is essentially the same. What you get is factory satellite (sorry, but I believe XM is the only option) with controls in the head unit and everything, including the antenna, beautifully concealed.
Fair warning: I did this myself in my 07 Touring and it is not for the faint of heart. You have to de-construct the entire perimeter of the interior, starting with the dash at the left of the steering wheel and ending with the right rear seat, sail panels and the rear deck. This requires care around all the airbags, several undocumented steps, mystery fasteners, and delicate trim parts. It's quite a sight when you (or worse; your wife) realize you've just disassembled your brand-new car and you're not really sure it will all go back together in time to drive to work Monday.
I've done this kind of work before and I admit this was tough - high enough on the level-of-difficulty meter that I recommend having a dealer do it. It's easily worth about $200 for labor plus peace of mind... and I still bet most service depts would try to short cut the job by mounting the antenna on the trunk lip or somewhere else undesirable.
So, consider an alternative...
Your 08 has an auxilliary input jack - yes? I can confidently recommend a handheld satellite radio with a dash-mount bracket. Benefits:
- choice between SIRIUS and XM (maybe not so critical, since the FCC just approved their corporate merger)
- a fully portable unit, probably with cool accessory options
- better controls & presets than the toyota factory head unit
- full channel/song display (the Avalon displays ONLY 10 characters of the artist/title info - no scrolling, no control whatsoever)
- less expensive out-of-the-box
- important: buffer memory so you can pause, rewind and replay, like Tivo
- definitely no installation hassle.
Good luck, and report back to the Avalon forum whichever way you choose!