If it has steel nuts/ washers and they are rusty, corroded can also cause poor connections that get hot----- and the internal connections between the head of the bolts and the shunt can get tarnished--- on mine I soldered the heads to the shunt, now ammeter cool
My friend has a 36 packard and it came with every kind of twilight bulb, regular, led, halogen, the previous owner Wes trying to get bright lights--- 4 volts to the rear lights, I traced it to the ammeter, the old nuts were used over---- 4 new nuts and now bright lights,