No idea. I made my kids sit in the car/shut the hell up and stay in seatbelts/car seats whatever. I am a mean mom. I make the dogs sit still too, don't care if it is a two hour drive.
I do have a really nice portable one I bought for hubby. I'd just go that route. You can take them and use them anywhere. I think mine is a sony. We hooked up some cheap computer speakers in the bathroom and he puts in a dvd when he was forced to sit in the whirlpool everynight for his bum leg. It was the only way I could get him to be able to relax and let the water etc do it's job.
So if the monsters can share, try the portable, about a hundred bucks but they are good for vacations, taking along when you have to do visits and want to send the kids off and away from the adults and all that jazz, plus you can hook them up to a tv if no dvd is available.