I had dealings with Sirius as well. I usually call to cancel before it renews and get offered the better deal. I end up paying $79cdn plus $20 royalty fee and tax, so a little over $100. I'm ok at that price but not for the $180 they usually want.
This spring I ordered a new truck which came with 1 year free. The truck would arrive near the time my current subscription expired. I got the renewal notification and called to cancel, explaining that I didn't need it because new truck came with it for a year. Two months later I noticed a charge on my credit card for $228 for renewal! I called to complain and get a refund. They say they had no record of my cancellation but promised a refund. Two weeks go by and still no refund. I call and get told again there is no record of request. Eventually I manage to speak to somebody competent and get a refund a few days later.
I think it always auto renews but they are required to notify you in advance. Always call to cancel and get a better rate.