OK, I had two days left on my current Sirius XM subscription.
I called the cancellation number on the website (1-866-635-5020). They had a five minute wait and was offered to have them call me back automatically. I agreed and got the call back in about 3 minutes. Talked to a very polite young lady and told her I wanted to cancel my subscription on one of my cars (I have the XM Select subscriptions on two cars). She said she was sorry to hear that and asked why. I said the service was too expensive. Within a minute she offered me the $89 a year deal plus taxes and fees (royalty fees). With a $1.65 credit for the remaining two days, my total charge for the whole year is $105.74 or less than $9 a month. After verifying my account and credit card information and 'agreeing' to have it renewed next year (although I may the same thing before then), the entire call took less than 8 minutes! No confrontations needed, no being a jerk. Just being matter of fact about wanting to cancel.
I saved about $80 off the previous price for about less than 12 minutes work! Call them. They will deal with you!