it is in the steering wheel or you may need to take the speed meter to access the horn
There are 2 horns on a Cayenne, both are located behind the outer front bumper air ducts. I never changed one but the factory way is to remove the headlights/front facial. From pictures I have seen, it looks like they might be accessible by removing the inner fender-wells if you have long arms and don't mine scratches/scraps. It would save you about 2hrs. of work.
The horn is located low in the center & just in front of the front wheels. You can gain access from the bottom of the car through a panel hole along the bottom or perhaps through a bumper hole. There are two horns and these are common failure items.
Carry a circus clown in the passenger seat and ask him to honk his horn as required
Replace the Airbag/horn bracket in the steering wheel, rubber bushings are worn out,
The horn is located in the space between the grille and the radiator support.
The horn is located on the steering wheel.
The horn is located in the engine compartment.
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The horn is located in the driver's side fenderwell
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The wiring for the horn goes from the horn to the relay then to a box located unde the dash which in turn connects to your horn pad on the wheel in the car
The horn is located behind the front bumper fascia