Mercedes 124 Anti Theft system

I have a 1990 300E with over 200000 miles on it. When I bought it from the 2nd owner he advised me that the Anti theft alarm had started going off randomly and that he had disconnected the horn but that the headlights would continue to flash. He instructed me to simply use the truck lock to turn off the flashing. That works but I would really like some input on how to dissable the alarm permanently. All help appreciated. Thanks-Ken
W124