As a frequent user of Sirius XM Radio I do have a way that you can take it between cars without any issues. If you have the stock Craig Radio, then you could use a cassette adapter, plug that into your iPhone and stream Sirius XM over there app.
This way, all you would be using is your data and wouldn't have to install any antennas. I've never had any trouble with doing it this way and think it's a better alternative than what the guy from Best Buy recommended.