Your car was available with either Halogen (H1) or HID headlights. With a yellow sticker stating "Xenon", I would say you have the Xenon lights as well.
Your issue will take some troubleshooting to find the problem. Easiest thing to do first is to swap bulbs from headlight to headlight. See if the problem sticks with the bulb, or if its still the same headlight with the problem, this will eliminate or verify the bulb being the issue. If the same headlight with a different bulb still has the issue, its something to do with the ballast or ignitor. Replacing either of these will require removal of the headlight from the car, replacing the ballast or ignitor is pretty easy, as long as you get the right parts.
Do the bulb swap test, being carefull to have the lights OFF when you're near then, HID operates at high voltage! The bulb connectors twist-lock onto the bulbs, they only turn one way then pull off the bulb. You can then unlock the bulb retainer ring, which twist-locks onto the projector, comes off the same way as the bulb connector. Certain vehicles require removal of the headlight to change a bulb, I am not sure if this is needed on the TT or not.
With all that said, give the swap a try... post back once you do with the result. If its a ballast problem, I will help you through it so you dont have to take it to the dealer. HID parts at the dealer are VERY expensive (try around $200 USD for ONE BULB!) If it IS a bad bulb, I can get you a new PAIR for $70 USD, always replace HID bulbs in pairs for best results and a match. If its a bad ballast, figure around $100 USD for the parts.
