In order to mount a sub you will need to cut an opening in the rear deck, both the metal and top cover. The magnet will only obstruct the trunks tension bars limiting how it opens and closes, so for the PITA it is going to be you might as well go with a powered sub in the trunk.
The BMW you saw with that had an audio system that was designed that way. They either put the third brake light on the roof in the upper part of the rear windshield rather than lower like on the G, or its on the trunk. Either way here in the U.S by law cars manufactured after 1986 are to have a third brake light, and I'm thinking where you are it's no different.
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