polarity is just active/active, neutral/neutral. Generally speaking with low voltage most things arent polarity conscious. I dont really know what you mean when you say a car can be on positive or negative current...
i was under the impression you had to change the reflectors...ie remove reflectors from another hid equipped car and remould them to yours to achieve the right spread of light. Half the difference between normal & hid is the wattage & spectrum while the other half is the reflectors themselves (correct me if im wrong).....anyway im getting off topic
got any diagrams or links to provide a bit more info