I've heard all of the horror stories, and I've experienced a few myself, but i just called them to cancel XM in the Vette and Sirius in the Audi and it couldn't have been easier, they even sent me refund checks for the unused portion.

I have a portable XM with a car kit that I'm going to use in the new X5 on road trips.

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You have your way. I have my way. As for the right way, the correct way, and the only way, it does not exist.