That the relay works means that the fuse, switch and wiring are all
ok. I would pull the horn wire and check it with a meter to see if it
has 12V when someone presses the horn switch. If it doesn't, the relay
or connections from it may be bad. If it has 12V at the horn, you
simply need a new horn (most likely). It is located near the front,
under the bumper area - driver's side, I think. Easy to remove.
If you can't find it, I will look at whree mine is - '01 ZX3.