well, a 50watt headlight and 4x3 watt turn signals and a 6 watt brake light is ~ 70 watts.
70 watts at 12 volts is like 6 amps.
This thing for $19 converts 72volts to 12volts at 10 amps
DC 24V 36V 48V 60V 64V 72V to 12V 10A Converter for Electric Storage Battery Car | eBay
But you will need one large 72v coil for the main contactor, and this would be powered when the contactor is closed (they can leak current)