The XM tuner is activated via its serial number with XM, can be done online (its very easy to sign up - its very hard to cancel). The subscription has nothing to do with the HU. The HU just lets you listen to the tuner. The tuners are sometimes activated by the dealers as a trial when new, if your vehicle is used than the previous owner had it activated and it will continue to work until their paid subscription expires. Could be a month to month thing, or they could have paid for a full year and you'll be using the remain time. It's also possible they paid for a life time sub and it will never expire. XM and Sirius have automatic renewals, a little sneaky, but unless you call them (no other method) and spend 30 mins on the phone your tuners subscription will continue to renew. If the previous owner doesn't know this and go through the effort of calling you could be listening to 'free' XM for a while.