Horn Constantly Sounding

Hi all,

First time posting. I have a 2010 Prius, roughly 150k miles. Saturday night after having just returned from a short drive I was sitting in the car, parked, but with the car still running. Suddenly the horn starting blaring. Not honking, sounding constantly at full volume (it was 11pm so I woke up the whole street). It sounded constantly like this until I figured out how to unplug the fuse box. I could not simply turn the car off to stop the problem.

Things I have done since:

• I checked the fuse box. It was not the alarm system as I removed that fuse and plugged the box back in only to have the horn immediately begin sounding again.
• I removed the horn fuse and that stopped the problem, however the horn fuse is connected with several other systems so I can't just remove only that fuse. The fuse didn't appear to be blown, but maybe I don't know what I'm looking at.
• I opened the steering wheel (safely with the battery disconnected) and checked that the plastic tabs that you depress to make contact with the horn were all still in place.

My current solution is that I have directly disconnected the horns at the horn itself, and now I can plug everything back in and drive just without being able to use the horn.

Mechanic (without having taken anything apart to check) thinks replacing the clock spring might fix the problem. It's a cheap replacement part aftermarket so I'm not too bothered by trying that solution, but does anyone else have any ideas here?

Additional details: Earlier that same day, we visited friends outside of the city and ended up needing to drive down a bumpy and muddy path. The brakes have been squealing at low speeds since then.