http://wac.addr.com/auto/obs/hud/hud.html

that's the headunit i used on my M roadster.

it requires you to tap into the speed sensor wire, tachometer wire, +12v, switched 12v, ground, and illumination wire, all accessible in the 3 connectors to the DASH/speedometer display console.

if you should go this route, i can look up the wires for you to tap..

http://www.defi-shop.com/product/vsd/vsd_top.html