it really is nice and simple

i have a jvc unit myself, nifty little things
The green cable that runs from the head unit is the earth as mentioned before, to make the dvds work all you need to do it connect the green wire to the metal of the car.
i found a bolt on the passengers side of my 6n2 that did this for me. To test if the bolt doe ground your head unit, i played the dvd so i could hear the sound, and literally just stripped the green wire and touched lots of different bolts inside the car untill i saw a picture!
then simply loosen the bolt that you choose, and then put the wire in between and tighten back up! make sure you strip the plastic casing of the wire mind other wise nothing will happen try to have about a cm of bare wire connected to the bolt
And thats it! done!
it also means you can be driving as scott said and watch your dvds at the same time, but we both know that people wouldnt be irresponsible and do that!
hope it helps, if you need any more advice then drop me a pm
Joe