Joe, I don't know if this information will be of any use to you but I own a vauxhall astra and had intermitant problems with the horn and as you say in your question, all the connections and the horn itself seemed perfectly good. I then received a recall notice from vauxhall who told me to take the car into a dealer where they would replace something on the steering columb because this problem with the horn had been identified. I took the car to the dealer and in no time they fitted the replacement part or modification. The horn has worked perfectly ever since.
John
March 2011
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