Quote:

when I run a ground wire from bat to negative head lite wire, light is bright with no fluctuation

I'd solder in a wire T'd to that neg HL ground wire & run it back to as close to the batt neg post (In that ground path) as needed till you get good continuity (& the batt ain't too far away) & going right to the batt neg post as you did when checking would solve it ASAP. or if you want to diagnose, keep checking continuities in the ground path till you find/fix the excessive voltage drop point & you likely would need it running so the vibration will make/break contact which is the light fluctuation you are seeing so you can see when you find the bad spot (poor/loose connection). I would connect a jumper wire with alligator clips to the batt neg post then touch the other end to the eng then to the frame then to the sheetmetal in the vicinity of the headlights till I find the spot that straightens em out

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