Had a look at http://www.madelectrical.com/electricaltech/onewire-threewire2.shtml
The odd thing is that all the diagrams of the 'three wire' only show two wires...
Is the third wire missing from the diagram for the idiot light?
The Lucas units I'm familiar with have 2 wires, one big fat cable to the battery/loom and one skinny wire to the idiot light (negative goes through the alternator frame).
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