It is my understanding that the shark fin is for BMW Assist as well as the Sirius radio. Don't know if 330i comes with BMW, but my 545 came with BMW Assist, so I got the shark fin. I also did the Sirius prep at the factory so that I could add later. With my car ordered in late December, Sirius was not yet being installed at the factory. In reading postings on other boards, those with later builds that came with Sirius factory installed, the shark fin is the antenna.
