My horn stopped working. I checked the fuses, and swapped the relay with the main beam relay to make sure I had a good relay.
I decided to take the airbag/horn pad assembly off and have a look inside.
The assembly is held on by 2 torx screws accessable from the underneath of the steering wheel, and whilst undoing the first one, I accidently touched the screwdriver against the ignition key housing. The horn went off! So to test the fuses, relay, BPM (Body Program Module, I think) and the horns themselves, just short out this screw to earth.
Of course the battery MUST be disconnected before taking off the airbag assembly.
Once I had the assembly off, I found that ALL four copper contacts (one for each corner of the pad) were corroded. Such is the price paid for leaving a 15 year old car out in the British weather all year round.....
Anyway, a quick clean with a small file, and all is well.