The paper insulator prevents the hot side from shorting to the plate; the steel washer on the ground side helps it connect to the plate, but I also moved my dedicated ground wire to the voltmeter stud.
This diagram shows the new R wire going to the ig.sw., but it's effectively the same as what's shown above (C223).
See also:
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https://www.fleet.ford.com/truckbbas/non-html/1997/c37_39_p.pdf 
