Here is some of the grounds to your headlights. Notice there are several of them. Now, how do these grounds make it back to the [-] side of the battery?

Thru the chassis, then the motor mount bolt, then thru the [-] cable? Or even worst thru the voltmeter negative and/or the temp guage wire and then thru the motor mount bolt? I hope not.

Hopefully you have a cable directly from your [-] side of the battery to the chassis.