I thought XM/Onstar was standard in all GM vehicles with a free year Onstar and 3 months XM if bought new.
You still have to activate them whether bought new or used. Some fleet sales and/or commercial vehicles may have been ordered XM/Onstar delete so if a dealer resells a rental company/fleet vehicle it may not have all standard equipment (some pre 2010 vehicles might not have the standard side airbags)
My XM radio quit yesterday, I changed stations, changed source, pushed buttons, no sound volume