After looking at the Prius stereo system with an Oscope I can tell you that it is different. From what I can see the head unit outputs left and right channels as differential 1v signals. These signals are not effected by volume or balance. This makes connecting the rio much easier! There is a second connector on the head unit that comes from the steering wheel. It has five wires and they are vol up / down, selection up / down and mode. There is a serial data line on the main connector. This carries the signal that controls volume. Volume, balance, fade, must be handled by the remote amp. When I am done the existing controls should be able to work with the Rio just like they do with the existing stereo. All I should have to do is install a 4PDT 12volt relay that is energized by the amp out of the Rio. This will switch the audio outputs of the head unit out and the outputs of the Rio in. I will also use the amp out to fire up the remote amp if needed. Not sure yet. More to follow. All in all connection looks like a breeze.

Oh yeah one last thing. I thought I would put the car back together without the stereo so I could drive it. When I started the car the touch screen was pretty much useless. No climate control, no nav system, nothing. It told me that it had lost connection with these devices. Reconnecting the head unit brought everything back to life. It looks like the data stream that controls all this stuff goes through the stereo. FYI