what would make the Beetle sercurity shut the engine off everytime you start it?

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The vw dealer told us it is the key but it isn't.The problem started since the guy is working on it started.He changed the heater core and cabin air filter.And was working on the fuses and then he said this problem started.PLEASE HELP the door windows quit on us after a guy put a new speaker in it so please answer what might be because the mechanic is working on it says he don't know what happened. Thanks Louisgal@hotmail.com