It is the top left relay in the fuse box.
THERE ARE TWO TYPES.
Later Version.
Pale grey with slots in, for two-tone warbler.
90 464 777
Earlier Version (and mk2 Cavalier / Ascona-C)
Plain black for mono-tone buzzer.
90 240 665
WIRING.
A quick look in the Haynes manual gives 3 inputs.
In order of the pins shown on the sounders above.
31b - Grey White = Ground contact via driver's door dip switch.
30 - Grey Green = Side / Tail light feed.
15 - Black wire = Ignition Live feed.
The black ignition-live feed appears to go to the illumination bulb of the light switch.
The grey-green wire goes to the illumination bulb of the other dash switches.
For the Grey-white, read below...
-----------------
FAULT FINDING.
If the buzzer has stopped sounding,
My theory is a loose, dirty or rusty connection on the driver's door dip switch.
Unscrew the dip-switch low down on the front of the driver's door frame.
It should have 2 sets of wires connected to two male spades.
A bunch of grey wires on one spade.
These are for interior light, electric window console etc.
The single grey white wire on the other spade is for the warbler / sounder contact.
It is kept separate from the greys, so that the buzzer does not sound when a passenger door is opened.
OPERATION.
It should only sound if....
Ignition off,
Lights on,
And driver's door is open.
REPAIR
It may be that the female spade / lucar connector with the grey / white wire has either become detatched or rusty.
If rusty, cut off the old one, and crimp, solder on a new one before refitting to the dip switch.