Replacing factory xenon bulb (2001 A4 1.8T)

Hey all. My buddy has an '01 1.8T and one of his xenon lights went out. I am familiar with aftermarket xenon kits (installed one in my Supra and have installed others) but I am not familiar with stock xenon kits. Can I just replace the bulb, or is there more to it than this? Alos, what type bulb is the stock xenon bulb, D2S? His have been very dim for the past few months and i assumed it was b/c they were about to go out, but i think he wants a diff. temp than what come stock, so I am wondering if I can buy any kind of D2S bulb (philips, etc.)? Thanks for the help guys.