1. You most likely do NOT have the cable installed. You can get one from forum vendor Tuan Lee
here
2. It would probably be sitting in the bottom of your center tub. To be sure you could pull up the carpet and check for a thick cable in the wire 'clips' above the rear passenger wheel well than runs to the tubs. It's easy to install one, but if GM decided to clamp it down in a place hard to get your fingers on it might get tricky to remove.
3. If he can remove the brackets it would be good to have them. My car didn't come with a changer, but when I got one I put in in the passenger side small tub and just wedged it in with some foam.
4. A good
DIY tech tip. It should be an easy "plug-n-play" install.
If you get your changer in the mail and it won't physically load the CD's from the cartridge into the changer I can explain how to possibly fix it. There is a 'pin' that goes in a 'tab' inside these changers that likes to pop out when UPS drops the box.