ok, from all the searches I've done I think I've got it figured how to add in my missing tacho loop.
the wire in the following diagram is the black with white stripe wire...



is it supposed to go to the fuse box like this? it looks like it's been spliced to power something else. my understanding is it should go from the firewall connector, up to the tacho loop & back to the ignition barrel in series like the dodgy diagram I did. Am I correct?

cheers for the help guys

Ash