Most USB adapters like this are in fully insulated plastic cases and have + and – terminals on the back. For a positive earth car just hook the + to ground and the – to a hot wither through a fuse as Tom suggests. This could be switched or un-switched, your choice. The device drops the 12-14v generator/battery voltage down to the 5v USB standard. You can search for these things on the web. There are even some with a built in voltmeter.