rear TV display (aftermarket)

[music824]

I recently did the Nav/DVD bypass mod for my wife and I also installed a 15 inch drop down monitor in the back for my son my problem is getting it to mirror or play the same thing that's on Nav/dvd screen as if it came like the factory ones where as though anything on the front nav/dvd screen can be seen on the back screen ? (Instructions please if it's possible)