I got a 5V usb power adapter for my car, but it broke and fell apart. I retrieved most of the parts, but the tip was lost. So I took it apart, and found broken wires and bad solder joints. I replaced the tip with an Anderson PowerPole connector. I ran the connector out the side where the ground lead used to be.
Also, note in the first picture that the red wire is used for ground and the black wire for +12v. I almost missed that. Be careful if you re-wire any of this stuff yourself, and trace the circuit!