I am sort of in the same situation. I have a GS and I have been looking into incorporating this thing into the factory unit for sometime now. To make a long story short, with Lexus, it can't be done.
Here are the problems:
The Lexus head unit and amp work together and are proprietary. The amp will only work with a Lexus or lex/nakamichi head unit.
The cd-charger is also proprietary.

You basically have to do what I am going to have to do. Change the amp and the head unit.
What I am planning to do is possibly get a clarion a/v setup with dvd and incorporate the riocar into it. Yeah it is going to cost a ton, but I think I can do most of the stuff myself with help from a couple of friends. A good source for Lexus “cans and can'ts” is www.clublexus.com.