Most screens have beside VGA input a regular video input, which can be selected on the screen,
My PS2 works nice on my ITX 7" TS, allthough I haven't installed it in my car.
Maybe I will some day for the back passengers, but I would use a separate screen for that.
Anybody knows if there are 12v-8.5V dc-dc convertors available for the PS2 in the car? Or is an inverter the best way to go?