Let's just assume that if you get the Satellite Prep, and later decide to also get the BMW OEM SIRIUS receiver, you don't have to install a separate antenna (which would be the whole point of the Satellite Prep option).
Ok, but if you go that route, I still don't see how you can avoid getting your whole radio unit replaced. How can the $495 Sirius satellite accessory from BMW be an external unit?? If it's external, how does it take advantage of the pre-wiring? Also, if it's just an external unit, how would it connect to your car and how could this possible be better than just buying your own Sirius satellite receiver in the first place? Thanks.