Modern Headlights

But did you also run a direct ground? Body grounds were marginal when new, and they don't improve with age. Also, lousy headlamps remain lousy even when properly fed and grounded.

which will require new sockets and terminals capable of handling the large-gauge wire, not the chintzy factory or parts store items that have 18ga.

Did you measure drop on both legs (feed, ground)?

Grounding is something I am always very aware of. I was in the computer power industry for many years..... Improper grounding, in a computer room environment, is the cause of 90% of all power related problems. IMHO
I have grounded the hell out of the entire car

I was going to rebuild the sockets if I can not get the wire gauge I want.