I believe it is a common fault & likely a manufacturing one. My Streetwise '04 plate did the same thing a year ago. When I was overtaken in the rain by a Volvo Estate, heading into oncoming traffic. I pressed the horn and it stayed on till I got home & removed the fuse.
Searched various websites looking for answer and they seem to vary. Most common one is that a ECU or something needs replacing. Prices quoted vary hugely, but older models before July'04 should apparently be cheaper to fix.
I've been told I'd have to pay £40/hr to specialist garage just to find fault. They didn't seem to know what problem was so I wasn't interested in writing them a blank cheque.
If you find answer let me know & I'll do like wise.
K Wilson
April 2008
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