Here's how I installed my DVD headrest.
I bought an Atrons 7" HD705 twin headrest kit.
http://xtrons.com/hd705-7-inch-in-car-headrest-dvd-players.html
The main reason I bought this model was that you could connect the cigarette lighter connector cable direct to the headrest under the hidden panel when you wanted to use it and remove it when you didn't instead of hard wiring it to the car's power and having to work out what power wires to connect to and dismantling the console etc to do it.
The other reason I bought it is that unlike many of the other headrests I could connect a wired headset to the player. It can like all the rest connect an IR headset, plus connect to the radio via FM.
Select your tool to feed the power/video cable down the back of the seat.
I used a plastic cable puller like the one below. (snake)
http://www.cablemagic.com.au/most-popular/yellow-tongue.html
I selected this over using something like a wire coat hanger as I didn't want it to puncture/rip the seat fabric as I was poking around guiding the cable.
The great thing about the HD705 head rest is that it has lots of plastic pole insert adaptors eg 14 and 16mm. Unfortunately the size you need is 15mm. So before I started I hand sanded the 16mm adaptor until it fit in the headrest hole. The positive side to this is that I could custom make the adaptor so it had a tight snug fit so the headrest did not move around and rattle. Nothing worse than a rattle right next to your head while you are driving. It did take me some time to sand. Once this was done the rest of the installation was pretty quick.
Steps
1) Tape string draw wire to the snake (cable puller) with electrical tape about 30cm/12 inches from the end you are going to insert in the seat.
2) Bend top of snake, about 15cm/6 inches from the end you are going to push into the seat.
3) Get in back seat and lower the seat towards you
4) Take out headrest
5) Push snake down seat hole which has the cable in the headrest
6) Poke around so the snake end goes between the seat gap and the rear seat fabric. It took me a few goes at trying the snake end at different angles. When you get it right you can see the snake end pushing on the fabric on the back of the seat near the top. When this happens reach up from under the fabric from the bottom of the seat.
7) Pull snake out the back seat. This will draw the string draw line through too.
8) When the end of the snake get near the end, ie about to exit the seat, slide the plastic adaptors over the headrest rods, tie the string to the end of the headrest cable to draw it through. Put some electrical tape to help too. Don't make it too big as it will get struck as you pull it through.
9) Pull the snake with the headrest cable into the seat and out the seat bottom. You make need some help to push the headrest inserts into the seat/push the headrest into place while you are pulling the cable through.
10) Disconnect the snake/string and electrical tape from the headrest cable
11) Put the end of the cable safely somewhere at the bottom of the seat in case you want to connect it to the cable adaptor which has the video in and out plus bar power cables
12) Plug your cigarette power cable to the headrest and to the power socket
13) Put in a DVD/SD card or USB with a Xvid movie (it was not clear like many of these type of players that the video format is Xvid.
14) Enjoy a movie.
15) Impress the other half on how comfortable the Focus seats are now and how good looking the new headrests are and not to mention the smile of the kids faces when they see what you have done.
16) Enjoy driving your Focus even more in comfort! No more back and neck ache!