It should work fine - it'll be 'looking' through the windscreen from under the dash anyway. It'll be easy enough to test when you 1st install the RNS-E and you can check how many satellites it's picking up in one of the nav screens. About 6 ought to be average - can always compare to Amar's RS6 RNS-E if it happens to be close by to check it's getting comparable reception for the location
I wouldn't bother with the box on the tailgate - it's just something else to leak etc if you retrofit.