I am having an issue posting the pictures here, but if you go back to my cardomain page you can see I added the changer and dvd player on page 3. And yes, any single DIN sized component will fit back there with slight modification. a little snipping of the plastic was all.. the DVD player that I used was single DIN and I liked the fact of it's practicality versus under the seat. I used a USA.SPEC cartridge cd changer designed to be used in conjunction with the navigation unit- here is one for sale on ebay
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=50556&item=5739191 347&rd=1&ssPageName=WDVW and Yes on-star, steeringwheel controls and low fuel
prompts are still functional with the current system. the cool part is that the dvd player audio can come through the system via the head unit RSE controls.. or switched off for headphones only. very nice feature.. Hope this helps and wish you and yours a happy holiday season.
-Tim