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the green/blue goes through the firewall to a box with 3 prongs on it next to the balast resistor
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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PS: this is an Auto shell - do they have different looms?
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.