Rallye cluster gauge issues - 67 Barracuda
PS. On my 67 Barracuda there was a ground wire from the capacitor mounting to the radio brace. It looks original but even if not, not a bad way to do it.
PSS.
Since Accessory also powers radio, turn signals, wipers, and heater fan, checking those will help isolate a loose or poor connection if that's the problem.