The Tach output is the opposite end of the 6A box to the main wiring and is a dedicated circuit. This the tach adapter should just plug in to the box there (with the tach adapter supplied wire, which may also be white)
The 6A box main wiring loom (with battery wires etc.) should have a white wire, which connects to the orange dizzie wire.
The tach adapter is an add on so if necessary just disconnect it for now to get the engine running
Julian
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