I didn't understand that you had two separate DVD players.
The GM audio systems have built in support for the following external items:
External CD Changer (CDX)
XM Radio (XM1 and XM2)
RSE (Rear Seat Entertainment) (Overhead DVD)
The DVD selection is for the factory overhead DVD players audio. The adapter I recommended tells the audio system that there is an overhead DVD player installed, and gives you that option on your NAV unit. When selected, it will give you the audio that is connected to the RCA jacks on the adapter.
Since you have two separate DVD players, some form of switching will be necessary. I'll try to do a little more research on this tonight.
Be aware that I don't do this for a living, but I did stay at a Holiday Inn Express.
