There's actually two buzzers. One for the key-in warning and one for the seat belts (time delay).
The key-in door buzzer plugs into the fuse panel in the upper right corner (is labeled buzzer on the panel) just above the time delay position, and is p/n 3780458 (according the parts catalog). Number on the actual part is 3747280.
The seat belt one is on the instrument panel reinforcement between the column and ash tray. p/n 4049059.
I've got a extra or two of the key buzzers. Check your PM.