This question is too general but will provide an answer to the rear light on 2007 CTS (which I presume is the same for all current models). The nuts needed to be unscrewed are accessed behind the body panels in the trunk. The body panel can be pulled away by unscrewing the tie downs (black plastic knobs) and moving the cover from the spare wheel well. Note: be careful unscrewing the nuts so they aren't dropped. Although the void in the rear corner is not deep, you will may have to clear some other components bolted to the body in order to get your hands into the void. Once the nuts are removed, the rear light cover can be pulled away from the body. The bulbs can be accessed once the cover is pulled away from car body.