[jmeckel]

Unless you are 100% sure of how your car is wired, use a volt meter to verify polarity first, or you can have a lot of issues including a destroyed charger and or battery. Or in a worse case scenario an explosion of the battery or fire. Take the few minutes to be sure. Any cheap meter will work as the engine should be off. Also take the time to verify your wiring and document any variation from the original diagram or change it back to original it will save a lot of time latter when something stops working. You will at least know what the "normal" state was.