Need to Silence Alarm for good

I bought a used 93 Park Ave about 2 years ago. Every once in a while when someone opens a door the alarm goes off with horns blowing and flashing headlights. I can silence it by turning the key in a door but I would like to silence it for good. Does anyone know of an easy way to do this??? It is factory installed and the previous owner didn't know how to disengage it either. Thanks, Mac