I installed Sirius in my '07 GS. The tuner does indeed get stuffed in the dash. That's a bit of a challenge, not a lot of extra space in there. It doesn't really matter where you place it since you won't see it again. I got mine on eBay but, if i did it again, would go direct to iSimple at http://isimplesolutions.com/ . The $220 kit came with an accessory input for standard 3mm mp3 player but i opted to get the iPod connection instead. This way i can connect my iPhone & direct it all from the head unit. NOTE: Read the directions on the order of making the connections. If you hook the line to the head unit before connecting the I/O it WILL NOT WORK and you'll scratch your head for an hour like i did. Just follow the directions & you'll be fine.
The only downside to this kit is that it does not come with a means to connect to the OEM shark-fin antenna. They supply the stupid little antenna thing that i REFUSE to put on my car. Just search eBay for TOYOTA/LEXUS SHARK FIN ANTENNA CONNECT CABLE XM/SIRIUS and you'll find it for around $60. The real trick is finding the end of the OEM antenna lead. I found mine in the trunk, passenger side, over the wheel well. You'll just have to be patient & go fishing around. Best of luck.