The port is badly broken, stop using it until you get it fixed.
If it has always been like this, chances are the port is miswired so that the +5V and ground pins are swapped (perhaps an internal cable plugged in the wrong way round). The reverse polarity would almost certainly destroy any devices plugged in (there's no way to plug in any USB connector the wrong way round, so this is not a risk that device designers would normally protect against).
On the other hand, if the port used to work normally and then started doing this all of a sudden, it's fairly likely the USB hub powering the port is toast and its 5V regulator is potentially letting full 12V+ through. Such an excess voltage will likely fry any unsuspecting device. The only solution would be to have the USB hub inspected and replaced. (Which might entail replacing the whole entertainment system or something, I'm not familiar with the car.)