Unfortunately, I think it started in 2001 or 2002 cars, where you no longer can get a code by pressing buttons anymore....you need a GM Tech II scan tool, to unlock it...then GM made it worse on the newer model cars.....the radios have the VIN number of the car programmed into it.....this means you can't swap radios anymore, because the radio will not be able to communicate with other modules on the serial data bus line....this also prevented dealerships from swapping radios in cars to make customers happy when buying a new vehicle....so basically they would order radios with the same options for their cars(the high end ones).....