Adding nav to 06 Miata, Need to find speed pulse at pcm

 

Hope you have a manual transmission and no ABS. If you do then then vehicle is equipped with a vehicle speed sensor that you will be able to tap into. This wire is Brown with a Red stripe at the PCM. Is the only Brown with Red stripe wire so should not be too difficult to locate.
If Miata is an Automatic or has ABS/DSC then the speed signal is picked up by the ABS module thru the wheel speed sensors and transmitted to the PCM and instrument cluster via the CAN network and you will not be able to get a signal for your NAV system as the signal is all scrummed together with a massive amount of other system information.