Did this today.
the back panel of the roof liner just pulls straight off and gives you limited access to the underside of the aerial. The aerial has 2 cables going into it and they have small tabs that pull downwards on the outer edge of the connectors and then they pull off. The aerial is held on by a T30 bolt and once you've undone that they aerial has 2 small black plastic tabs that need gently squeezed and it pops out.
The replacement comes unpainted, however the cover is held onto the base by 6 T6 bolts that once undone, allow the cover to come off and be swapped over.
Under the cover was soaking on my original aerial, I applied a thin coat of clear silicon to the cover edge before putting it onto the new aerial base.
£102 later and job done, sat nav now knows where it is.
Poor piece of design by Blaupunkt who make these aerials methinks
