My old chaps 1.5 DCI has a very intermittent horn. A lil while ago he had to have a new ecu or whateva cintrols the wipers coz it had f**ked up. They took the steering wheel airbag cover off and dont seem to of put it back on right. If u locate it properly the horn dont work, but pull it out slightly and it works fine.
This is a problem coz its his driving school car!!!!