Some vehicles come with convenience buttons on the steering wheel that control some functions on the radio like volume, seek, mute, etc.
Not all vehicles come with these buttons but my findings conclude that the radio's firmware accept the commands even if the buttons are not present. On vehicles equipped with the buttons, when presssed the small computer inside the button housing generate and send commands over the GMLAN CAN bus and the radio interprets them.
My vehicle does NOT have the radio buttons, only cruise control buttons.
To demonstrate this, I wrote a little Android app that communicates with my Can Bus Triple that emulates these commands respectively.
For added coolness, I made the Can Bus Triple send the radio's display contents to the app :)