How do you unlock the anti theft radio in a 1999 gmc sierra?

i dont think you can. It has a anti theft device in it so it only works with that one truck. it will learn the VIN of the car, truck, etc. and then when you try to put it in another car, truck, etc. it wont work. as far as i know 1) there is no way to unlock it and, 2) only GM has this.