Thank you, and your welcome

The color codes are :

POWER

• memory........blue
• ignition..........grey
• ground..........brown
• illumination...green
• dimmer.........white / black
  

SPEAKERS

• Right +...........red
• Right -............green
• Left +..............white
• Left -...............black
  

I used the illumination wire to dim the Rio. It is a full 12v and cycles with the headlight switch. The dimmer wire is variable with the dome light thumb switch.

I am not sure that I have the polarity of the speakers correct. These signals are differential on the Prius. I based + and - on Oscope readings and color data from a generic Toyota wiring list.
If anyone has a Toyota wiring list for the 2004 Prius it would be great to know if these colors are correct or not. It does seem to sound correct.
You are correct in your assumptions that the volume control on the steering wheel is the only control that effects the Rio. The mode and up / down buttons still work with the existing stereo only. This buttons do provide discrete inputs to the head unit. It would be quite possible for someone to add another relay and port one or all of these switches over to the rio. Using a couple unused contacts on the Rio connector and modifying the Rio. It would be possible to make them work. If there was one function I would like to have it would be pause. I may look into this after I have enjoyed my new car and the Rio for a while. It was undrivable for a week while I made the Rio upgrade.

CYA
C