Hello, and can you point me in the right direction?

Hi all. I've been eyeballing the Prius since i first sat in one 8 years ago. Well, being of a "pay as you go" budget, we went looking at one car and came home with a 2002 Prius. It came in 3K below bluebook and other than the color (a periwinkle silver) it looks great.

The first thing I thought I had found is that there was no disc in the navigation system. But as I got into it today, I realized there was but it gives a read error "wrong disc" message. I had taken the unit out, disassembled it, cleaned it and restored it to its place in the car...still no joy. I entered into the secret menu to try and reset it and now am stuck in a Japanese wrong disc error screen.

Secondly, as I got into the manual and set about programming the radio, the audio button doesn't seem to work. All the other buttons do, just not that one and Toyota says there is no button replacement, just the whole stereo for $600.

Had I known more about the car I might have ferreted these things out but the dealer didn't even know half the stuff on it. So I guess the questions are these, is there an after market stereo system or nav system or a good source for salvaged parts? Better yet is there a fix for the audio button?

Thanks and God Bless,
Kyle