When you say "DVD player" are we to assume that you mean some sort of monitor and dvd player whether it be a combo unit or seperate pieces?
Either way, there options for all-in-one overhead monitors and headrest systems. You can purchase pre-fabbed headrest systems that have a monitor/dvd in one side and a slave monitor on the other. Downfall is that they are usually only covered in leather AND if you camp (i.e. sleep in the E) you want be able to use them while in camp mode. Overhead systems have the advantage of not only coming in larger sizes, but they can be used in camp mode. It's all personal preference really.
You'll also want to consider how the viewers are going to hear the source. Is it going to play through the OEM audio system, an aftermarket system or independantly (through headphones) of the vehicles audio system. Again, personal preference, but the headphone option allows the viewers to listen to the movie and the front seat passengers to listen to another source at the same time.