Has anyone here been inside a stepper motor driven tacho?

Here is my current problem, trying to get a tacho to work in my work car. Originally it had no tacho, I've grafted in a gauge cluster with a tacho and have everything working except the rev-counter.
It appears somewhere during the 20 hours of sorting wires I have fried something or it just refuses to accept the signals I am sending it. Now the tacho on this cluster is driven from a stepper motor with only two fields. These fields check out resistance wise with a multi-meter, so I'm convinced it's the driver circuit in the board which either has issues or just doesn't like the signals I am sending it.

I don't want to install a seperate gauge tach, because that will look silly and waste space for guages that are actually useful. So I'm hoping if I can find a gauge which is stepper driven (I expect they all are) and has two fields driving the stepper then I can simply run wires from inside the gauge (which will be buried in the dash) to the factory tacho and it'll possibly work nicely.

So, anyone done something similar?