If I were you, I'd put it in the sun visor in place of the mirror, either in yours or the passenger side. You could also buy another one and put them in the headrests depending on how many people you have ride back there if any.
If you really wanted to go wild you could actually install a 4" monitor into your steering wheel and hook it up to a navigation system and/or a license plate mounted camera that automatically comes on when you put the car in reverse. That way you could also have navigation literally at your fingertips.
