Hi Kai1234. with my problem the bulb stopped working but it was indeed the ballast unit that had broken and not the bulb. If you want to diagnose, you should first swap the bulbs left to right and visa versa. If the same side still does not work then it is the ballast. this is located under the headlight and the only way that I could see of getting to the unit was by removing the bumper / front fo the car. Its not that bad to do once you realise you have no choice. the ballast itself is expensive. I was quoted 330 euros but got second hand one for half that. The reason that they fail is they are exposed to the weather and eventually it gets into the high voltage circuit.
In terms of the heated seats, I still have the fault but came to the conclusion that it is probably the switch in the seat - hence not related.