AIR HORN will not work...........in my letterbox

[XFISH98]

Dear brains trust............I have a car air horn, that is not being used it's its conventional way of being in a car, but in my letter box. It is solely to be used to blow, when anybody but the postman goes near my letterbox. I have run the wires from my lounge room , through the front wall, under my pea gravel front yard and up into my letterbox. Now, When I hook it up directly to my spare battery, it will work as it should. But I have gone through JAYCAR and they suggested a simple transformer ( 240V / 12 v ), that i could attach to my wires, and nothing happened. I returned to the shop, then they suggested this.........12VDC 12.5 Switchmode Power Supply - Mains to Cigarette Lighter Socket https://www.jaycar.com.au/12vdc-12-5...ocket/p/MP3573

After wiring it in, I get a small /short 'toot' noise, the machine turns off...turns itself back on, gives another small 'toot' and turns off ( and repeat ). The wire I have run is 10 meters in length and in a 3amp gauge. Thinking, my horn was buggered again........i attached the spare car battery again..........and the horn works properly. What am I missing to make my horn blow. ? Agreeably, this is a very unconventional way to wire up a air horn.......but I get sick and tired of junk mail, and all the other **** that gets fed to my letterbox. I have a 'no junk mail' sign on my letterbox, and has been there for years..........any idea's? I have a security camera over my front yard, that show's my letterbox on my screens, hence when I know when someone in near my letterbox. I have a wi-fi power plug, that I can turn the power off/on via an app from my phone.