Try doing a search here and on RennTech. I have heard of this happening before, I think it was caused by the wire in the steering wheel coming loose over time. It isn't to hard of a DIY.
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1999 Ocean Blue Metallic Boxster - blueboxster.com
If you are fat, maybe it's your belly hitting the horn button :-)
While I know its not your problem, here's an amusing little story:
I once had a car that had the ignition on the steering column. I also had just a few keys on a plain keyring. Every once and a while the other keys on the ring would flip over the top of the ring under hard braking. If they flipped just right one key would flick into a gap in the plastic around the steering column. It would then short out the horn button and the horn would come on and stay on. The first time this happened it took me about ten minutes to figure out why the horn was going off :-(
Since I don't have the code for my radio, I didn't wanna do this fix... especially since it includes air-bag removal, which to me is quite scary. So I just removed the horn fuse.
However, now when realizing how useful a horn actually is, I wonder if there might be some other way to install a horn... like like a air-horn that I can perhaps control remotely so I don't need to pull wires and stuff...
...or any other suggestion appreciated.
Since you've already gotten a bunch of good advice, I'll offer some less than helpful advice:
Don't brake...
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Or he could just put a "Horn" label on the brake pedal and call it a day?
I used to be into wakeboarding and read about a guy who had a boat that whenever he hit the horn the engine would actually increase speed/power. Evidently there was some sort of electrical draw on the engine with the horn circuit. He fixed the problem by putting a "Turbo" label on the horn.