VSS location on the NC is a big mystery. Non-bose versions are supposed to have it at the radio, but I have never heard of anyone connecting one. I guess most of them are Bose equipped.
In the Bose equipped cars, the VSS wire is not even installed in the radio harness.
there are several discussions about this on the forums at miata.net.
If you do find something, I'd love to know about it. That being said, I have been using my D3 without the VSS connected for over a year. Other than parking garages and tunnels, I can't say it's really been a problem.