[adam911]

That's a great help, thanks. I shall just reconfigure it to miror that and see if it will work. Oddly though, I cannot find a ground tab on the tach, just the three tabs in a row surrounded by a white plastic block on the back. The white plastic block is actually big enough for 4 tabs, but there are only three protruding from it. Would one of these be a ground?

Here's a picture of the back of the Tach as I removed it. You can see it has the Blue and yellow wire and the black wire connected (that ran the length of the car and connected to the coil) and there was nothing connected to the third tab. Sat behind the tach was a ground and the black with voilet tracer just sat unconnected to anything.

I should add, my car is also on Carbs. I'm assuming the speed switch is related to MFi?

Adam