I went and got some ebay ones about 2 weeks ago- 9" headrest DVD installed them into a ML triton.
Cost a little over $300- then added rechargable AAA batteries (all 10 of the things- infra red headphones and remote controls) and 2x 32gig SD cards.
The SD cards will be installed with about 50 movies (compressed format) which should mean no scratched DVD's in the rear (Kids are 5 and

don't quite trust them yet.
Instalation-
My main bugbear was getting direct power from the battery, and getting this power into the cab.
I bought 5 strand trailer wire and went in through the dust cap (main wireing feed on the passenger firewall) but it was time consuming with me being home sick- genuine and having little paitenance)
I went with direct power supply due to concerns of loosing power if stopped for fuel etc- this way the dvds will still work with the ignition switched off. I went with the 5 strand, as I can add a 12 outlet in the future. I have a 6 blade fuse block at the battery for accessories (uhf, anderson plug, dvd, spots, and some other things that require power)
The triton seats were easy to slip the wires down, I did undo one of the wire circular holders at the bottom of the seat in order to get my arm up into the rear of the seat. I have them fully connected together so either can be a slave or master or independent. I did not however join them to the front DVD player. There is a huge bundle of wires now wrapped together against the siderail on the passenger seat (about 10cm diameter x 15cm) I had hoped to "hide" these under the centre console but no such luck due to the size.
One concern is that they draw constant power- red led light on at all times, but the draw would be minimal- just got to be wary that the power button does not get pushed while the covers are on.
The screens look to face the roof a little where IMO should be angled slightly back, but the kids are not complaining- and that makes us in the front happy as well, will insert a switch into the line to isolate power, to minimise the kids use if required.
Been installed for 3 weeks now and have had no problems with DVD's skipping or anything, so far.