Hey Jared,
I double checked the depths of the two Clarion units and
dave mix (mcaddyman) is correct, the XMD3 is shorter than the CMD5.
So I can't install the CMD5 because the steering wheel bracket under the dash limits the depth of the head units.
MB Sports changed the location of the bracket or something else in '06 because the CMD4 (or CMD4A) is the factory installed head unit.
I did compare the wiring on both Clarion's and they looked very similar, so that part might not be too tough. I'm not sure about the cutout in the dash.
I am going to install the Clarion EA1251 iPod interface and the LCD remote in the transom when I find some extra time to do it!!