My '03 silverado 2500 has weak hi and lo beams on the drivers side only.

Voltmeter says only 4.5 volts on that side to lights. Pass side is bright and reads 12.7 volts.

I suspect a ground issue but can't find where the wire loom might go to ground. I also tried ground the bulb with a jumper to the chasis but it did not seem to brighten the bulb.

Any ideas what might weaken the voltage on drivers side only???