First of all the alarm and vacuum locks are activated by a micro switch on the vacuum element, and the convenience feature is activated by a switch on the door handle itself. The convenience feature is separate from the alarm. I have yet to run into a 90 or newer 124 without this feature although I can't swear it iwas an option. You can check easilly by removing the bottom section of the rear seat. The control module is located under the seat on the driver's side.

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