As ever, you're a superstar!
Aye, found the destructions.online... got remarkably excited when I saw *simulated air horn".
I am a child.
When I first fitted it, the Omega was still a Taxi, so I had the Airhorn only wired in to sound via the factory horn. The delay in the relay meant that you got a the car horn if you blipped the horn button, but if you held it more than a second you got the full 100 Watts
When it came to the Plod gubbins going in completely, the horn ring controlled the siren cycle once the noise button was pressed on the controller... Basically you press Siren/999 on the controller and the siren switches on. You then use the horn ring short press to cycle the siren tones or a quick double tap to silence it. Doing this leaves it active so if you press the horn again it will start up again.
Although you probably know that from work
I modified the wiring to the alarm switch (plod cars have a blank fitted here) and used that for the Airhorn. This only works when the siren is switched on/armed on the controller.
If you wanted to be really fancy, you could wire it so main beam would cause the siren to kick on with the headlight flasher relay
Pm incoming