So, I had one of these boys laying around:

And the user guide ofcourse..

http://archiv.me-mo-tec.de/default.aspx?ID=28&t=Bits, at the bottom you can download the german one (I also have the german one).

This thing comes with 4 wires, one of them needs to be going to the ECU to measure the RPM. Does anyone know which wire that is? I'm planning to mount the shift light above my speedometer. Any help appreciated!

Orange wire from the ECU to the ignition coil is the one you want.

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Awesome, thanks!