Certain beckers (eg 87 Grand Prix) don't have a code anti-theft feature, at least not the one that requires that you put in a code when the 12v is disconnected (they WILL if the anti-theft wire is disconnected when the alarm is active).

What happens is that the internal battery dies and then when the 12v is disconnected, all the presets disappear and the radio ceases to function. Then you have to have the battery replaced and the radio reset by a diagnostic machine at the factory or dealer.