Unfortunately, my understanding is that this is a dealer replaceable part, the front bumper needs to come off. Definitely not happy when I read this, but hoping that it doesn't happen more than once while I own the car. I think it sucks that it was designed this way. I actually may have thought twice about buying the car. Don't get me wrong, I love my CTS, but it ticks me off when I may have to spend big money to change a $10 part.
The front bumper has to come off then the headlights have to be deinstalled so you can get to the rear of the headlamp module where the bulb access covers are. It takes some time but its not actually that hard to do.
have done the procedure myself last week twice without taking all the front bumper off. the trick part is getting into the screw thats secure the side bumper on either side of the wheel, then after have to "bend" the side bumper enough to get to the third and final screw that holds the light assembly. to take the assemby off pull the lower part and push the upper part and it will came off.