Rich, I have the NAV package...so my second slot (hidden behind the tilting display) is for the NAV CD (or DVD, whatever!).
The only other (exposed) slot is for EITHER CD (as in music) OR DVD (as in rear seat entertainment). This means I CANNOT listen to music from a CD, while my kids watch a movie in back. They have the wireless headphones, so I can listen to AM, FM, or XM while they watch their movie OR I can listen to the movie, BUT I can't listen to my CD while they are watching a movie - it's the same slot.
As I said before, in my wife's car, the CD goes in the front and the movie goes in the back. With wireless headphones in there as well, I am free to choose from any audio output (including their movie), simultaneously, while they watch their movie. Sounds like a no-brainer to me. All I know is that in a truck this big, it couldn't possibly have been a 'space' issue.
But I still love 'er!!!