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Connect a cigarette lighter socket (female) direct to the battery as negative ground like a modern would be. This creates a separate circuit that is negative ground. The side of the socket (female) is negative ground.
Use the cigarette lighter plug and wire that came with the phone or GPS to charge or to use like in a modern.
The cigarette lighter plug (male) from the phone or GPS only lets five volts pass through. Phone and GPS are five volts devices as are others that are meant to plug into a cigarette lighter socket.
Check your work. Disconnect device when not in use.