I was amazed that the a class ,recently purchaced would not open when I pressed unlock with my key. I opened it manually and , the alarm went off, It would not start. alarm kept going on. I dicconnected negative terminal on the battery. alarm went off eventually. this is my first time having a merc. I replaced key batteries. connected the battery, O joy ,alarm went off again. off with the negative terminal again What is going on. jim77