How do you remove the back of the nav housing? Do I have to remove the whole nav unit to remove the back of the navigation housing?

If you mean the back of the hood - yes you have take apart the "hood" assembly that rotates on the motor and remove it from the gears. There is 1 small screw on each side, once you get those off then you just gently pry it off... there are just small clips holding it on.

Thanks.