I wish I had the electrical engineering skills to design such a box. I can solder it once someone designs the layout, though

I think all newer cars have a wire like that (most electronic speedo's use it). A complete wiring diagram should display one wire as VSS (vehicle speed sensor), at the least going to/from your ECM (engine control module - computer), if not for varying other uses (speedo, auto trans etc.). The problem in having one box for all cars is that some cars use 8 instead of 4 pulses per revolution. That's why routing it into the empeg would be nice: software control of the pulse reading!

The wire in my Miata is a dedicated wire just for the ALC - and now it's just lying there, bare, unemployed and depressed :P

Besides volume control, one could use the speed info to have the empeg play sounds once you reach certain speed(s), change visuals, calculate distance (if the odometer is broken, or you don't have/want to use your trip odometer) or simply for display.

There is a diagram in one of the threads (or links therein) to convert such a pulse train into a serial format. I'd solder the thing if I knew I had someone to code a program that could use the input (I could use my laptop to check functionality) in the empeg. Any takers?

Edit: Ooops, just noticed Richie's post. Must've come in while I was writing mine. Your Miata only has the wire for the radio, if you had the MPSS or MSSS from the factory (M-Edition cars, C-Package etc.). It's unlikely that you had it on a base model. Nevertheless, dependng on the year, somewhere to/from your ECM you should find the wire with a proper wiring diagram. I found my wire using the Miata MSS FAQ.

What year is your Miata? You can find some wiring diagrams online at http://www.madracki.com/miata/wiring.html.

That'd be awesome if you could design such an interface! Be sure to check out the threads I linked in my post above for some fairly detailed info.