My quick read of the installation instructions for the harness "nukerj" listed says that the harness may not be what you want. It appears designed to allow the Miata to tow a trailer. I could be incorrect.
For helpful reading,here's a link
to a Google search of iRV2 posts with the term "Miata" in them.
I'm all for plug-and-play products (although I still customize). Sometimes there just isn't enough market for those products, so people have to cook up their own solutions. I'd do up my Miata the "Healeyman" way, if I had one.