Just wanted to add to this - horn was broken in 1.8 Sporting, FIAT prices are insane so followed the instructions here, bought a silver horn thing from Halfords and it worked fine. First checked the fuse and that the relay was clicking when the horn was pressed, to minimize the chance of an unnecessary DIY job!
Had to cut the connector off and use spades to hook the wires up to the connection on the new horn though, couldn't work out how you'd managed to do it without butchering the connection HWJTD? Kept the connector just in case but I guess it doesn't matter, working horn better than non-working horn!
The other slight difficulty is bolting it back, as the sizes are different. Ended up bolting the metal plate that came with the new horn to the old one with the right sized thread & nut, as the hole is too small (or the thread too big on the Halfords one). Aside from drilling a bigger hole, not sure how else to attach it (any ideas?) but it'll do, seems secure! Only spent about £17, so hope that might help someone..
