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Your fuel gauge should have a plasic cap on one of its studs to prevent placing the connector on it. That connector does belong on the 12 volt supply to the mechanical limiter which is inside the fuel gauge.
Please keep in mind that a chassis ground is required. Nothing about the instrument panel will work properly while dangling from its harness connectors. Screws in the panel to the dash provide chassis ground but adding a dedicated ground wire is recommended. Happy moparing