Tony,
Thanks for the link. I'd gone through that FAQ a few times but never picked up on the link to the other thread until this morning.
At least I know I can fix my Rio.
I'm still curious as to why the problems occured. For now, I'm running in forced DC mode through Hijak in the other car that's had the Rio in it for a few months now. I know the wiring in that car is correct with no blown fuses.
I remember the sequence of things a little better now that it's morning though. I powered the Rio up the first time with no problems, however, the amp turn-on lead had come disconnected before power-up. This posed no problems (other than no sound) and the Rio shut down automatically when I turned to key off to fix my temporary wiring. It wasn't until after I corrected this problem and powered the system up again that the AC/DC problem occured (along with the screaming speakers and fried amp).
The switched accessory lead from the car worked to turn on the amp with no problems before installation of the Rio - no blown fuses. I realize that the output of the Rio for amp power-up is very low current, and if the amp was damaged, it could have drawn too much. Is this circuit connected to the AC/DC switching circuit? What conditions will cause this diode to release the magic smoke?
I'm just trying to solve the puzzle - I will be swapping amplifiers to a known good one, but I don't want to take the risk of frying the Rio completely, or taking out another amplifier, or both.
Thanks again for the help!