Hello ' I got a 1995 E320 Mercedes . The driverside wont lock with the key .It seems like its lock ,the little pin on the door goes down indicating its locking ,but if I open the door the Alarm goes off and door opens up .All other 3 doors lock fine .It happened about 6 months ago .At first it was intermittent but now it never locks .has anyone encountered this problem and would tell me what it is please ?