Sorry for the very late update.

I got a engine issue which put my car off the road and engine out time ( weird knock noise, we think it's a spun bearing, stopped the car immediately and trailer'd it home. Here's hoping no damage, just a new bearing )

Any ways, I got to the point where the tach would work- sort of. It wouldn't pickup the signal at Idle speed, but would read RPM when revving it, so it was always behind 800-1000 rpm.

I Measured the resistance in the signal pickup sensor wire and it's too much out of spec, so i need a new pickup wire.