I checked with miata.net and found the following:
The Miata's speedometer pulsed signal is generated by an internal switch to ground (if I recall correctly, 4 grounding pulses per revolution of the speedometer cable).
Is there a way to use that?
BTW, here's an interesting thread about the same topic from about a year ago.
Is there any news on microphone use for either volume control via ambient noise or voice recognition? I found some discussions on the board, but nothing definitive...