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Sounds like the horn relay.

Tacho wire is BLACK/WHITE. It is located in the fat loom directly behind the tacho mounting position. From factory it has coloured tape holding it in position.
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ALL 1200 looms contain the same wires. From factory the tacho wire is located in the same position on all 1200's. I have noted in the past 1200's, 120y's and 200b's all have the same tacho wire from factory mounted directly behind the tacho mounting position. All vehicles having this wire located with coloured tape.
( Thats from factory ) If someone has removed your BLACK/WHITE wire, it will no longer show as stated above. ( I cant see anyone having removed it though )
Even if you still can't find it. The first link you posted, contains info on how to correctly wire the tacho.

Cheers for all your help phunkdoktaspok.

someone must have buggered with my loom by the looks of it. I'll rip the dash out of my parts donor shell & see if that loom is fully intact.

both looms have been butchered, ####it

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

yes thats the way it is ment to be. But if you simply run a wire and leave the other wire still powering the coil - the tacho wont work.

Easiest way to say to do it is;
Take the +pos wire off the coil. Join a length of wire to it. Run it up to the tacho and through it. Then run it back to the coil and join that end to the coil +pos.
Simple.

rockin, thanks Steve