I believe there is a security relearn procedure if you haven't tried that already I would try it with the gt pcm. security relearn can be annoying but the radio could be the cause I snagged a radio out of a junkyard and it wouldn't let me start the car unless I took it to the dealer and got that relearn fixed so you may have to do that. but instead I just got a pioneer with the interface adapter ended up being like $10 more than if I got the dealer to relearn the used radio and IMO the pioneer radio looks better and for some odd reason makes the whole system sound WAY better. much more crisp clean sound. so if I were you I would see if your old radio allows the GT pcm to start and if it does you can keep it that way, relearn the new radio, or get a aftermarket with a good interface (bad interfaces cause problems).
