Installing a DVD changer can certainly be done, however it requires a fair amount of modification. Most changers are too wide which means you have to take out all the electronics (CD changer, amp, nav computer, etc.) in the trunk and make a custom mount bracket that will hold everything. I believe this is what X5monkey did.
I installed the single DVD player from TVonNAV which fits perfectly next to the nav computer. It comes with a small remote eye which you can mount anywhere upfront to control it via a small remote.
I also used the NAV-TV harness (similar to the TVonNAV harness) which allows video in motion, reverse camera, and DVD in stereo through the aux-input. The DVD player easily plugs into the aux-input directly into the radio module under the spare tire. That way you don't have to run any cables to the front of the car or take apart the glovebox.
I am selling my DVD player. If you are interested, send me a PM. I have used it *maybe* twice since I've had it. Still got everything in the box. I just don't seem to have the need for it. It is a DVD/CD/MP3 player. I will post details in the 'For sale' forum later...