HID Relay Harnesses Explained - Better Automotive Lighting May 15, 2012The primary function of an automotive headlight wire harness is to provide power directly from the vehicle’s battery to the lighting system. This allows the bulb, or HID ballast, to operate independently of the vehicle’s original wiring system and can provide a power source that is more consistent. A lighting relay harness is most commonly Author: Betterautomotivelighting Amazon: hid wire harness Set of 2-9006 HB4 12V Female Wire Harness Connector HID Light Wiring Replacement Pigtail Adapter Plug Socket Cable. 3.8 out of 5 stars 14. $6 $ 6. 99. FREE Shipping. Only 8 left in stock - order soon. 2 Pieces set 9005/9006 Headlight Bulb Xenon Halogen HID Light Connector Wire Harness.