Ptwat, in order to get the unit to sit flush with the radio bezel to make it look factory. You have to cut the plastic trimming in the back of the vehicle once you take out the factory radio. When you take out the factory radio, you will see that the plastic housing will go from side to side in the back as if it was a one piece of plastic molding. Then right behind that housing is another piece of plastic (black in my vehicle). I cut both piece off with a tin snip as I see that it wouldn't affect anything, doing that made it possible to slide the 120bt all the way back to make it flush with the front radio bezel. Do a search on this particular radio on this forum and you should see some pictures of different types of install. Hope that explains a little why you want to cut the back of the housing off, any other questions feel free to ask.
Sorry about not answering your question fully. I have a 2010 Yukon SLT. What I mean by "cut the front piece". Once you take the factory radio out, run your hand from both side to the back (or visualize with your eyes). About 1" before you get to the back part of the housing on both side, you cut right there, so that you are basically taking out a letter U. It sound hard but took me a little more than 10 minute to do once I figured that a tin snip was my best tool. Using a razor blade I score a line on both side, again 1" right before the bend. Once I scored a line, I cut with the snip and was able to snap it right off.