I noticed the horn on my GT had stopped the other day, and after consulting this forum I checked the options.
A quick look under the bonnet revealed that I couldn't get my hands down in the available space without dismantling a lot of other periphery, so I cut straight to the circuit board.
I positioned the steering wheel to give me access to the three 5mm Allan screws and took the key out before dismantling. Everything came out without a hitch, and on examining the circuit board I could see no evidence of corrosion at all. Having dismantled, I went ahead with the bypass operation anyway, and on re-assembly the horn worked again. Great piece of information, but got to say I'm at a loss as to why it should fail in the first place!