I started down this route but changed my mind, looked at all the generic ones that are around which seemed ok but read conflicting reports on some of them regards rattles and things and looking at branded ones i considered an Alpine unit but the price goes drastically up.
Was never convinced on the idea of having a drop down across the rear view vision (which is already not great) so settled on two headrest mount units that can link or be independant, only put them in when we go away and have headphones etc so kids can watch separate films or link together and share one DVD, really pleased with them now and glad we got them
Not an answer to your question but my take on it