I am not sure if your head unit allows playing/watching video at all, but of it does, you need ground the brake wire from the unit and thats it. You will be able to watch video while in motion.
Well, it will go against the law, but be smart and don't watch movies while you drive, it will safe your life and life others.
Regarding to the rear passengers, if you add headrest monitors or central video, there is nothing to worry about, just do not connect audio to your stock speakers, let them use the headphones.
The best stock video headrests made by Invision, the cheap chinese all will do work great, however buy it directly from manufacturer. I have both and like both as well.