Thought this might be useful for others as this ought to work for ECU's other that MS too. The scenario is that the stock GTV6 tach doesn't work on a signal direct from some aftermarket ECU's.

This is because the PLAIN WHITE wire from the stock revcounter / tach is connected to the negative coil post where the tach receives a short duration high voltage spike for every ignition event in order to give a useful readout.

I'm using 6 coil packs running wasted spark and the tach can't get the right signal. A circuit (3 diodes, 3 resistors on 3 coils) can be made up to give a suitable feed - but I couldn't get this to work.

However the following circuit does work, it just uses the small coil within a standard automotive 12v relay to simulate the short duration high voltage spike needed. It effectively runs a low power coil as if it were a small ignition coil). I've put mine in a household metal junction box (faraday cage interference shield) in the engine bay, but it's small enough to go anywhere. Oh, and open the relay and remove the switch as otherwise it chatters at low rpm!

Hope this is useful.

I've not tested the idea, but there are cheap little PCB mounted relays that have the required drive circuitry to be triggered by a 5 volt signal. For only a few dollars and a compact surface mount design, I'd give this a go.
Look for 'Arduino Relay' on ePay.