My daughter has a ka 07 the horn didnt work, fuse was ok so i went and got a horn from a scrappy.
Could not find the horn anywhere, so went on line and asked where it was located. lots of crap answers but a few pointed me in the right direction. I can confirm that the horn was behind the passenger section of the front bumper. (not accessable under the bonet)Firstly I removed 3 screws on the plastic panel under the passenger wheel arch this allowed me to pull back the panel and i could see the horn. I did not remove the wheel to do this. I then parted the bumper below the headlight this made it easier to remove and replace the horn. hope you find this useful.
chris stevenson, February 2011