If your Nuvi was designed for car use then it will have a power lead that plugs into the lighter socket and the usb connector on the Nuvi. The lead drops the car's 12 volt supply down to 5 volts for the Nuvi. If you put 12 volts directly into the Nuvi it will die instantly. Wire in a socket somewhere that rain won't fall on it and use the Nuvi's power lead as Garmin built it.