[dmsfrr]

The ground wire between the engine and body on a '55 / '56 is often overlooked. It may be located between the engine and; firewall, inner fender or radiator support. Some may also attach to the frame.

The 'ground' battery cable in a '55/'56 is usually bolted to the engine to make a low resistance / high current ground connection for the starter.
Because the lights, accessories, starter solenoid and voltage regulator all mount to the body, if the body-to-engine ground connection is bad it messes up everything.

The one in this photo (removed from my '55) was apparently undersized enough to nearly melt.

IIRC, for '57+ the battery cable ground connection was changed to the body. But a much larger ground cable between the body and engine had to be added to carry the current necessary for the starter. (along with 2 more high current connections that can get loose or corroded)