If the switch is a single wire one then yes it earths through the switch and master cylinder, But I thought (again going by old mechanics memory) the XF had 2 wires?
I know the 540 I am building has 2 wires.
And a big Hoorah for the little parts that make a big difference
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Cruising in 5th
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Never be afraid to do something new, Remember, Amateurs built the Ark: Professionals built the Titanic.