Alarm Problem
 

I have a 1991 350SD. A couple days ago when I unlocked the door and opened it, the alarm started sounding. I started the car hoping it would go off, but it didn't. The alarm will go through it's operation and when a door is opened the alarm will start sounding again. I disconnected the battery for a few hours and that didn't help. Please tell me what I can do to make it stop.