Re: 156 horn permanently on
It's fixed, and it was an interesting one!
During the couple of week's I drove around with no horn, I noticed that a relay was clicking when I put the steering wheel on lock, which sounded the same as the one that clicked when I pressed the now silent horn.
I followed David C's suggestion and looked behind the airbag. The entire airbag is mounted on a steel plate, that is spaced away from the magnesium casting that forms the steering wheel by some springs and guides. This is the spring action for the horn 'button'. To get electrical separation between the two, the 4 springs are held on 4 plastic washers moulded onto the spring plate. What had happened, was that under the influence of time (7 years) and temperature, the two uppermost plastic washers had cracked in half, which (by a strange piece of design) releases the spring force that holds the steel plate away. When the steering wheel was on lock (as I must have left it in the carpark all those weeks ago), the weight of the airbag was enough to push the horn button itself.
My fix was a bit agricultural - I replaced the springs on the two affected bolts with plain steel washers and did them up. The horn works normally again now, you just have to push at the bottom, and it won't affect airbag deployment.
Very pleasing. I think more people will see this failure mode as their cars age.