One of thre three pins on the back of the sealed beam is the ground. Use a set of jumper wires and a battery to wring out which blade terminal is the common ground which allows the high and low beams to burn when power is applied, and trace that wire in the 3 pin plug to the point where it attaches to the body metal. Be aware that decades of rust can interrupt the normal pathway back to the battery, so adding a few extra ground wires can definitely be to your advantage!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!