I can't answer your question but be advised that this really requires you replace the amp and all the wiring also unless you go with something pre-packaged (bavariansoundwerksdotcom). You can also try the BMW M audio subwoofers with existing amp (search this forum)
I went the second route and you'll be amazed how some bass improves the overall sound quality of the system - gets rid of the "mid bias" of the stock system.