Even for normal bulbs I always build a relay harness for my lights. On my Jimy through the factory wiring I had 11.7 volts at the headlight. after building a relay harness I had the alternators full 14.3 volts at the bulb. made the silverstar Ultras I had in it REALLY bright and white. For HID I would definitely reccomend the harness too. gotta get the proper voltage to your lights.