Should a relay be used to power HID, yes and always needs to be used. Why you ask perhaps? Your oem halogen equiped car was never designed or intended from the manufacturer to use or run high voltage/high current/ high amperage HID ballasts. Ballast draw a imense amount of amps upon start-up and could very seriosuly damage your wiring and not just at where its connected. We are talking serious damage to fuse boxes, ecu's, or worse could short and cause fires on very old cares that even have a hard enough time trying to power halogen. The reason why is, that when the ballast "demand" power, your car has to supply it from somewhere. Lets say its tapped into your oem headlight wire ok. Now you power up the ballasts, the draw current from your wiring, your wiring might not be up to the task so its needs help, t searches for a source and before you know it, you've now weakend not only one source but two now just to try and supply the ballast good clean power. This is why a relay harness is needed. A relay harness gets its power straight from the battery via relays. These relays are then wired to go to your ballasts now.