Here is how I sorted out the issue of playing DVD's and getting stereo sound.
There are three key components involved:
1. Video in/out lead for the TV tuner (or aux in/out at the rear of the car)
2. TV in motion module
3. Aux input for the stereo
The DVD needs to be connected as follows: DVD video output to the video input of the TV tuner (via either a TV module cable or aux in/out at rear of car). The DVD audio connected to the stereo aux input.
The smart piece in the puzzle is the TV in motion unit.
The one I used allowed a seperate audio source to be selected whilst video is displayed on the monitor.
The way it actually worked is by pressing the <> button on the monitor toggles between the Video audio (i.e. mono) and the last selected audio setting on the radio. So, by making sure the radio was set to AUX input, pressing the <> when in Video aux mode produces stereo from the DVD player.
FYI, this is how I installed the AV setup in my car.
Using the audio video switcher, each of the three monitors can select a video (and associated audio) source independantly of each other.
However, this installation wasn't pain free. I had an unforseen issue of the DVD remote interfering with the video switcher, so in some cases pressing play on the remote changed one of the screens video selections. Also, in the UK, the TV video input is PAL and the DVD unit I got doesn't play PAL too well, so I've stuck with NTSC as it produces the better picture on the headrest monitors (but as a result I don't get a picture on the Nav screen now - but the kids are quiet so I can live with that!)