When trouleshooting, first thing i always do is this and it seems wierd at first:
Connect voltmeter's negative lead to a good point on the body.
Connect positive voltmeter lead to battery's negative post, that is push lead against the lead post that the battery cable clamp grabs around not the battery clamp itself..
Have somebody turn on headlights, meter should read very slightly positive...
More than a few millivolts means i need to clean up connections at ends of negative battery cable or fix engine to body ground wire.
On old Fords look for somebody replaced negative cable and didnt realize that the little metal tab in middle of original factory cable WAS their negative connection to body for both battery and engine. Melted braid ground strap at back of engine is a giveaway..
A natural enough mistake, it looks innocuous.