There are several warning buzzers:
key in ignition
lights on (88 - up only)
These buzzers can be unplugged or even other ones plugged in.
Only the 88.5 and 89 XR's have the headlight warning. They changed the wiring slightly to add this to the seatbelt buzzer. I retrofitted my '87 to add this. Only requires a single wire and a 5 cent diode.
There are two physically separate buzzers, one is called the 'Dual Warning Buzzer' and houses two buzzers. The other is the Turbo overboost buzzer. The Dual warning buzzer is used for the seat belt reminder and the Ignition warning (Door open, key in ignition) I believe that this one is also used for the headlamp reminder on the 1988-89 cars.
The dual warning buzzer can easily be swapped for a chime module from a Taurus. It is the exact same connector and is a 5 minute job to change it.