I recently learned of an XM Radio promotion that extends your subscription by one year for $60. I don't have a promotional code but just asked about the "Additional year for $60 deal".
When I called, the first customer service rep said I would have to call back once my current 3-month free trial ended. However since I was worried that the promotion might expire prior to that, I called back a little while later and the rep I spoke with this time said no problem and that my service would be extended by a year after my free trial ends.
I believe that the promotion is limited to XM acquired through new vehicle purchases but I'm not positive about that. The phone number I was told to call by the initial XM rep was 800-998-7900, which I assume is the Promotions Department.
You will need to provide your XM Radio ID which you can get by tuning your radio to Channel 0.