We've got a roof mounted DVD player in my wife's minivan. It came with it when we bought the van.
One thing you need to be cautioned about with these automotive DVD plays is the ridiculously high cost of many of these sets. The roof mounted versions can cost up to $1,000.00 and the headrest versions can go upwards of $600.00.
The issue is these things are many times very unreliable and extremely expensive to repair. In fact, most of the time a repair will cost as much as a new unit.
On the other hand you can buy a portable DVD player for as little as $100.00 or even less. You can mount them to the headrest. Also you can buy one with two screens or you could buy two completely separate units. This is handy in case you've got a 4 yr old girl wanting to watch My Little Pony and a 10 yr old boy wanting to watch Transformers.
Bottom line is before going and buying anything, do your research. Read up on the different options and make an informed decision.
This post was edited on 2/5 at 1:23 pm