The "factory" dual headrest system is not an in-dash player. The DVD players are built into the headrests. Your in-dash unit is just a CD player unless you ordered the factory ceiling-mounted DVD system.
Per the 2008 and 2009 ordering guides (could not find data for 2007 model year):
US8 is the base radio and will not play DVDs.
UVA is the non-navigation radio used with the overhead DVD player and will play DVDs.
U3R is the navigation radio for vehicles without the overhead DVD. THESE UNITS WILL NOT PLAY A DVD![/color]
UZR is the navigation radio for vehicles with the overhead DVD. These units will play a DVD.