The SPID is a sticker. It went on the car at the factory and either survived or didn't. Where it lived changed by year:
1982–1986: glove box or center console. 1987–1990 GTA: center console. 1991 GTA early build: still center console. 1991 (late) – 1992 GTA: rear hatch storage compartment, sometimes called the 'rear glovebox'.
Many SPIDs are missing because owners peeled them, dealers removed them at re-trim, or interior reupholstery took them. Many are partially legible because they fade in the sun, especially on convertibles. When the SPID is gone, the door VIN/data plate is the next-best cross-validation source — less code-rich, but it confirms what the car was.