Curious why you would do this, the head unit for the radio is under the Nav not behind it, do you intend to have two head units? Also won't you lose function as the head unit currently talks to the NAV system to update display information, although most people probably don't put their NAV into audio mode like me, and I only do it when listening to baseball because it shows the score. I guess the thing would be to ensure that the depth of your new head unit matched the CD unit pretty well, there really isn't that much space lost back there in comparison to Ridgelines without NAV.
I wanted to take out the factory radio and replace it with the pocket (that's in it's place in the non-navi models) and take out the cd changer and replace it with an aftermarket head unit for a nice look so you can't see the radio and still have the ability to put cds in the pocket. Thank's for your reply anyway.