Most kits come with wiring adapters that let you plug your factory headlight plugs to the ballasts to supply power to the ballasts.
Bad late night ascii diagram:
==[]-----[ ballast ]-------[] []-----------adapter harness------------[] []---------your stock headlight wiring----bulb
Some kits come with a fused relay harness that allow you to feed power to the ballasts direct from your battery or alternator, but use your factory headlight wiring as relay on/off trigger. This is preferred. HIDs are only 35 watts, but the ballasts draw more current than halogen bulbs. They need a good, steady, supply of power. This method would place less strain on your factory headlight wiring.
Example:
http://www.virtualvillage.com/...46633
Some even add a diode to prevent current surge and reversing... which could fry your car's circuitry.http://faqlight.carpassion.inf...n.gif
Modified by feloniousmonk at 1:09 AM 6/25/2008