Pulled out the old CD/radio/cassette player in favor of a new unit that does it all (okay so I lost the tape player but gained USB, BT, and AUX)
The unit needed a special wiring harness to keep the stock Bose amp and speakers.
And that needed some further modification! Thank goodness for miata.net and thousands of other people who have already figured this stuff out.
A few minutes of troubleshooting and wire swapping and I finally got it running.
Cram all the wires in there and go for a cruise with all kinds of tunes!