how to bench test a 70 Dart instrument cluster?

I don't know if a lantern battery is stable enough under current. I'd use a car battery

Ground LOL. NO this isn't a crystal radio. The ground means the NEG side of the battery. The cluster is originally poorly grounded to the car dash/ body/ column support "which is" battery negative. So you add a pigtail to the PC board screws (traces the traces, they go to the ground side of the illumination lamp sockets) Add a wire, and that is your "battery neg"