For the moment I have a 93 MPV and it shares the same stereo as the Miata from similar era.
Both the Miata and the MPV have an automatic selecting dual-antenna system called "diversity" system and your factory unit should have two antenna connections. A female and a male connectors coming from the dash and a matching pair on the factory stereo.
I installed an Aiwa headunit that I had lying around and annoyingly, reception is noticeably inferior to factory stereo, but it doesn't surprise me as the antennas aren't put to their full potential. Some stations that were clearly flawless now have statics.
Any Miata people faced the same problem? How did you fix it?