The 2008 CTS is the same as my 2009 CTS.
To access the any of the bulbs, you need to remove the bumper/grille assembly.
I just did mine and installed some aftermarket HID bulbs.
If you have some skill and patience, it will take you around an hour.
I'll try to remember all the steps, but I did find a video on youtube that shows it very well:
Remove the long plastic cover over the radiator, by unscrewing the 6 plastic clips. There some rubber molding attached, but you can just flip the cover over on top of the engine.
This exposes 2 10mm screws that can be removed - they'll look like they are attached to the grille. Don't remove them yet.
If front of each front tire in the edge of the wheel well, there are two plastic pins. The pins are actually two pieces, pry the center piece straight out about a quarter inch, then the whole pin can be popped out.
Under the front lower edge are 5-7 similar clips that need to be removed the same way. There about 15-20 of them under there, but you don't have to remove them all. I recall that two of them are upside down.
Now he scary part.... back to the wheel well. Get a flashlight. I recall the inner fender is under the bumper edge; it needs to be pulled out from behind the bumper. Then pull the inner fender backwards from the bumper; it's very stiff, but you need to pull it about 3/4" and look straight up through the gap. There is a 10mm screw, about one inch from the edge of the bumper/fender. This screw holds the bumper to the fender. Use a 10mm socket and a long extension to remove it. (do both sides)
Then you can firmly pull the side of the bumper sideways from the car.
There are 4 of those plastic pin/clips to removed under the hood, next to the headlight. Then you can remove the two 10mm screws under the hood, and the bumper can be lifted off.
Each of the headlight housings are held on by two 10mm screws and a 10mm nut. The is one electrical connector to unclip.
There are 3 covers on each housing. Each cover is held on by 3 8mm? screws. Your low beam is the top bulb.