Doesn't happen during normal driving, but today I drove hard into a corner and when I braked hard the horn sounded. (scared the #$%@& out of me!). It doesn't happen when sitting and pressing the pedal hard, only when slowing hard from speed. I'm thinking something is flexing and shorting the horn circuit, or something weird is happening with the ABS (seems like a stretch). Any ideas?

 

Maybe the spring in the horn button on the steering wheel behind the airbag (or buttons depending on which steering wheel) is bad...and sudden deceleration closes the circurt..heck I would swap the relay before looking at the wires.

 

Wow.... just when I thought I had heard about every existing MINI malady.... a new one. :-)

 

I would trace the wires from the horn back as far as you can looking for any damaged insulation on the wires. Sounds like the wire is shorting out against something when the suspension compresses during braking.

The spring in the steering wheel is an interesting idea. Does the horn button(s) feel loose when pushed lightly. Is it very easy to honk the horn with the button as if the spring is very weak or broken?

 

Problem solve! I took the airbag off the steering wheel and found that one of the three springs (the bottom one) that supports the horn mechanism had pushed through the rubber piece that holds it. That left the horn contacts almost closed. All it took was a bump or hard braking for the contacts to close and the horn to honk. I cut a piece of rubber to keep the spring from pushing through and all is good. Glad I started there rather than taking the front of the car apart to chase wires!