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"Door-ignition"? I think you mean the keyswitch is no longer working. Your local M-B dealer can help you, if you give the chassis number, they will get you a lock which you can fit or have fitted, and all will be well again. The procedure for removal and fitment of a do00r-lock is given in the Haynes manual for Mercs, if you are brave enough or want to buy the lock from your dealer, and do it yourself or contract a mobile-mechanic to do it.