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How are the headlights grounded?



Dunno

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Headlight switch?



Dunno

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The engine block has a ground to the firewall?



The block has a ground to the battery (-)

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How about the instruments?



The instruments ground through an extra opening in the bulkhead connector and attach to a wiper motor nut/stud. My bulkhead connector may be bad, as could be the grounding point on the wiper motor, I suppose.

But why wouldn't they all have issues then?