While at the dealership the other day, I read a section of the RX-8 service manual which described an onboard diagnostic function for the audio system! You're supposed to be able to enter this mode by the following procedure:[list=1] Turn the ignition switch to ACCC or ON.
While pressing the POWER button, simultaneously press the AM button and the TAPE/MD button for 2 seconds or more.
[/list=1]Apparently because this only works if there is actually a problem somewhere in the system - if a code or codes are displayed, they indicate a problem with one or more components. "If no fault is stored, no codes will be displayed." When I try it, nothing happens. Wish I'd known about it when I had the faulty head unit that wouldn't display MP3 tags.

The first digit of the codes, if any, identifies the manufacturer of the problem component: FMS Audio(1), Panasonic(2) or Clarion(3).