The Pantser

Why not just add a new switch or button to the car? Like a button for your left foot to hit like old headlight high beams were.

You may be able to use a optocoupler to sense the horn then use that to trigger the relay. What are you trying to accomplish? Are you making like sensor to know when your car alarm has triggered?

Those look like they put old flic buttons into a shell. Speaking of Flic button might work too. They are Bluetooth and support ifft and other protocols

“What doesn’t immediately kill you always makes you stronger”

Cancer?