welcome aboard stevenzil!!
i have a NAV system in mine.
the DVD is in the trunk. you will not need to change out anything like with the CD's from the previous systems (pre-2003 models).
behind the DVD screen in your car, there is a CD player. all you need to do is press the eject button and the screen will slide down to access the CD player. the CD Changer is not needed, but can be nice to have especially on long road trips.
the DVD player in the trunk is only for the NAV system. it will not play the DVD movies. if you want to play DVD movies, you can get a system from NAV-TV. their website is http://www.nav-tv.com
hope this helps.