I'm not sure what you mean, the horn button is labeled HORN right on the face of the unit. Are you talking about the horn-ring function, where you hit the horn button on the steering wheel and the siren or airhorn sound? It has that, and I used it in the M1009 but will not use it in the HMMWV; I like the cheesy meep of the stock horn, and sometimes it's a better option.
The Whelen will also route the radio speaker output to the siren speakers so you can monitor the radio while you're away from the vehicle; I'm not using that function at the moment.
I have a SmartSiren (stashed in the garage) that uses a remote faceplate and it features several aux switches that can be used to switch lights or whatever. I programmed the siren sounds to be nonfunctional when I had it briefly installed in my CUCV in case a cop wanted to make an isue of it. I'm keeping that unit in case some cop does make an issue of it since technically sirens are not allowed in private vehicles in WA. I'm hoping the lockwire that disables them in mine are good enough, but if not I'll swap over to the SmartSiren which cannot sound a siren tone without extensive reprogramming.
I have about 80% of what I'd need to install a real big-rig airhorn setup (the 3' long horn, tank, valves, and solenoid powered valve) that's also very loud, but the siren unit is far easier to install and gives me PA if I have something that needs saying loudly.
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