Theres several things that come to mind
1. You may want to check the wires hooked to the door contact switch make sure they are not broke just squeeze the top and bottom to the switch and pull give the wires a little yank to make sure they are sound.

2. Contact on the buzzer its self may be dirty locate the buzzer under the dash and spray the connectors with some contact spray or carb cleaner.

3. Buzzer may not be hooked up if... if you bought the car used someone might have disconnected it. sometimes when buzzers are shot they wont be quiet and keep on chirping. sometimes they dont work at all mine buzzes but it is very faint due to the underdash cover

Thats just a few ideas I've used when running into that problem in the past



Jeff