forgot where this connects (picture)

That is definitely NOT a gauge voltage regulator - the regulator (when external) is a small rectangular can with two straight, flat connector 'legs'. A regulator is connected in series with the supply to limit the voltage. You show that capacitor (noise suppression is likely) connected as a parallel ground. I suspect it should be plugged into the back of the ammeter to limit noise from the alternator being picked up by the radio.