64 Dart cluster connector

The original instrument regulator/ limiter IS INSIDE THE FUEL GAUGE if it has 3 terminals.

I agree likely ground. Wiggle/ work the cluster connector to scrub/ clean the terminals and feel for tightness. Remove the connector and examine the pins to the board for looseness or corrosion. Clean and solder them, use liquid / past ELECTRONIC soldering flux if need be. NONE of the flux sold at welding/ plumbing or Lowes/ HD is electronic compatible. It is all acid

GROUND: Examine the board, and find one of several mounting screws that is also common to a grounding trace and add a wire pigtail which you can bolt to the column support or the dash main frame. These are originally grounded via the cluster mounting screws.