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I'm keeping the 6V -- all the equipment is period correct including the "bread box" radios that go in the trunk. The siren light atop the car sucks down a whopping 33 amps at 6 volts when engaged. You have to run a relay off the battery via a momentary switch on the floor. It's like the old high beam tilt/ray switch, but with no "catch".
We had a 64 International Travelall that had been an ambulance when I was a teen, the siren was the same type. There was a toggle switch on the dash that would switch between the horn and the siren, when running the siren you'd hold down the horn button and it would wind up until you let off it was an awesome thing to play with.