Would these work?

[Snipeye]

Or even just one of them, really - look carefully at the pictures that show both screens, you can see a yellow, possibly composite, cable going into the second screen.

http://cgi.ebay.com/Polaroid-PDM-2727M- ... 35ae65dbc7" onclick="window.open(this.href);return false;

If so, pretty cheap for 7".

[Biased]

If it's like the grand majority of retail in-car DVD player systems, my guess is that the secondary screen has a composite video input. So yes, at least one of the screens should work (or be easy enough to wire the video and audio for).