My 2004 cavalier stereo is locked.i put a

That's the anti theft lock... It's not a system that comes with every car radio, but i've saw a few of those on cavaliers and sunfires, and a few civics, and i think a couple others have some too, but i've only personnally seen and worked with the ones i mentioned. In any case, unfortunetly even if i know how they work, there is no easy way to get them unlocked and working again. You need the special code for that specific radio, and to get it you don't have the choice to go to you GM dealer and ask them to do it. You need their special machines to get the codes and get the radio unlocked, there is no other way other than changing it for an aftermarket radio again like before. So if you really want to get the OEM radio going again, go with the car to your GM dealer to prove to them it's not stolen and they will get the codes from the GM support center with their dealer code and will unlock your radio, but of course they will charge for it, dealers never give any gifts to anyone.:p So ask them how much they charge for the service, cause in some cases, buying an aftermarket radio will cost less.