Hi,timallen,
My technicien need the service manual to be able to repair the radio. I have the wire harness for the Miata radio, those plug are the same for 1990 to 1997. I plug those wiring harness to the radio with a 12 volt charger to get power, evrey thing light up, cd, cassette,radio am/fm; can change stations on radio,radio accept cassette and cd , reject when a push the button; but no sound come fron the unit.
I print the 18 pages from the service manuel on your link, also I send this link to my technicien hoping that will help, waiting for is respond.