Well, i had actually just replaced both of my bulbs last weekend. My driver's side was intermintent, esp in the morning. I bought the factory lamps off of rock auto, they were $144/ea plus shipping. It has them listed as an OSRAM xenarc lamp from germany. I then pulled off the front bumper and pulled both headlight housings off. I find that the easiest way when dealing with the front end lighting. I mean the whole bumper is held on with many plastic clips, 2 bolts, and 4 keyhole slots. It really is a lot easier that it sounds, i think it takes less time just to throw the bumper off than wasting time to find ways around it.
ANYWAYS i got caught in the middle of the job because the back of the housing is held on with anti-tamper torx screws.(the torx screws with the raised dot in the middle) You can get a cheap set from advanced. ALSO, to my surprise, my factory (?) lamps were labeled as Phillips Xenstart. The osram xenarcs came in a ac-delco box, and as far as i know osram-sylvania is a seperate company from phillips. Im sure a gm parts guy would know the answer to this.
To make a long story short, I through the new ones in and put everything back and havent had a problem yet. Im hoping the ballasts dont go in the future, which by the way are labeled as AL from france. Which makes me more confused because the replacement ones im seeing have a different part no. and are germany. Anyway they look great also, better than my old ones, they have a really 5000k whiteish color to them that i really like from Cadillacs.