do you know the security code? because if you don't know the security code you'll have to take it to a dealer. you should be able to input the code right when you turn the radio on, it should say something like "input security code" or something to that effect. I know thats what happened with my stock tape deck.
you might try pulling the battery cables OFF the battery and touching them together, this might reset the radio to factory specs, and make it "forget" the code, but I don't know, it's worth a try......but please make sure the battery cables are off the battery before you touch them together or else you'll have more problems than just the radio code!