awg

First thing to try, test the headlight direct from the battery with either positive, or positive and negative wires, at least 10 Amp..does it shine bright now ?

Then do it with positive only, if it shines bright, your ground is ok

If not, time to break out the test light and/or multimeter, I prefer a test light for headlights and such, trace the circuit back with the test light

Some PO on one of my XJ installed an overly elaborate system of relays to operate the headlights, five in all

I had a couple of issues with it, but diagnosed and fixed them with my $5 test lamp

A multimeter can be handy to diagnose any suspected high resistance or lack of continuity

I probably should get in there and rewire the whole thing, except it does work, and I have other jobs that should be done first