This video talks about Classic Holden/HSV, but apparently affects trucks/utes quite a bit also. I guess because these are tall, and easier to get access from underneath.

Tip of the day: car thieves reach in wheel well, and cut wire to horn, so that only lights are flashing when they break in.

So install a second horn to your existing horn relay (if possible) to make it harder to steal.

I checked and horn laws are pretty loose in New Zealand. You can't use a bell, siren or whistle, and a vague statement that "It should not make an unnecessary or unreasonably loud, harsh or shrill noise."

How to Wire a Second Horn to an Existing Relay

1) Check the Relay's Current Rating: Look at the specifications of your existing horn relay (often labeled on the relay itself or in your vehicle’s manual).

Ensure it can handle the combined current draw of both horns. For example, if each horn draws 5 amps, the relay should support at least 10 amps, with some margin.

2) Wire the Second Horn: Identify the wire running from the relay to the existing horn (typically the "output" terminal of the relay).

Splice a new wire into this circuit and connect it to the positive terminal of the second horn.

Connect the negative terminal of the second horn to the vehicle's ground (chassis).

3) Test the Setup:

Press the horn button to ensure both horns sound simultaneously.

If the relay trips or does not work, it may not be able to handle the increased load, and you'll need to upgrade the relay.

Classic Holden/HSVs being stolen daily using this cheap eBay tool

CarExpert