I've was researching this topic obsessively, attempting to retro my '05 tC with a new iPod head unit and I'd thought I'd share my finding s with everyone.
My solution was to score an '08 xB with base stereo, but it sounds like many others are trying to retrofit their older rides with the heads.
You'll need the following:
Scion iPod Head Unit (All units that support iPod inegration will work)
Console Cable (Plugs into factory wiring harness, then into console. Provides "jack" for iPod cable, PN PT546-21061)
iPod Cable (Plugs into iPod, then into console, PN PT546-21062)
Lastly, you'll need a cable that integrated into the factory wiring harness. This cable is nearly impossible to find, until now.
A seller on eBay has made a custom version of the integrated cable that DRAMATICALLY simplifies retrofitting a Scion model that doesn't include the iPod wiring harness.
The custom cable plugs directly into the iPod head unit, then into PN PT546-21061. This will allow you to finally get your iPod connected properly into your Scion head unit.
You can find it here: http://cgi.ebay.com/ebaymotors/Scion...14190066QQrdZ1
And there you have it! All the pieces required to retrofit ANY Scion iPod head unit into ANY Scion vehicle.
I hope this helps those who've been struggling with a solution...