We went with a Panasonic unit. It has a mounting plate that straps to the headrest posts. Then the player itself clips into the plate. I highly recommend this style as opposed to something custom fit to a particular vehicle. Makes it easily usable with multiple vehicles and comes in handy if your destination doesn't have other forms of kid friendly entertainment.
The Pannie's battery is rated at 10 hours on a full charge with proper dimming of the display. Using default video brightness with no effort to conserve power, we get 6-7 hours of real runtime. Makes a convenient time limit on how much video time they get on a long road trip, but there's still a car adapter if you need it.
-Brent