The wiring harness is at the bottom of the headlight enclosure. A quarter cup of water inside the enclosure is enough to short the bottom of the harness and cause low beam failure. Emptying the water and drying off both ends of the harness (QTips and bits of paper towel) are enough to bring the low beam back. Step 5: Test mount the projector with all the pieces needed to hold the bulb in. If there's not enough thread exposed on the rear, check to see where the projector is hitting the housing and sand it down or cut it. Step 6: You can leave the projector in there from now on. Next up is resizing the shroud. Bi-Xenons are Hi and Low beamswork just like OEM except 3 times brighter and whiter. The bulbs have one Xenon filament with a shield that partially covers the filament in Low beams, then the shield moves - electrically - to uncover the entire filament to full beam when you turn on or flash the Hi beam switch. Works great. Hope that helps From what I understand, all US market GTI's came equipped with bi-xenon headlights. You are correct. The "bi-xenon" means that the same HID bulb is used for both low and high beams; the difference is the position of a movable shutter inside the projector assembly. Your car is telling you the HID bulb is out. You need a D2S HID bulb. When compared to traditional H7 halogen bulbs, bi-xenon headlamps provide up to 86% more luminous power when in low-beam mode, rising to as high as 181% more while in high-beam mode. This enhances visibility and vision, Bi-Xenon Headlights. which offers significantly safer driving at night-time. Additionally, on some vehicles, the cornering Stefan. #1 Apr 12, 2015. Dear fellow F-chatters, I need some help. Just came home from an easter trip to Italy, driving 3.000 Km with my 612 without any problems, just to realize at home that the right head light isn't working anymore, both low- and high beam. Fuses are all ok, so I guess its the ballast or the bulb or both. Most of the confusion surrounding headlights is the difference between single-beam bulbs and dual-beam bulbs. Briefly speaking, a single beam headlight system uses two headlight bulbs, one for the low beam and another for the high beam. On the other hand, a dual beam headlight system uses one bulb for both the low and high beam. In very rare I installed a H4 Bi-Xenon HID conversion kit as others have done. This kit uses only one of the original headlight halogen bulb plug socket, leaving the other original plug socket free and not connected. All worked as expected except that the dash high beam indicator lamp no longer functioned. vQq1g.