sorry i mis read your post.
in that case do you only want to do it when its in reverse?
if so you could run the video signal wires from the mirror down to the screen. use a video switcher so you can control what you see while in reverse. now if you really want to get tricky you can get a split screen module so both can be displayed side by side at the same time... i have seen it before but dont know what they used.