First, disconnect the battery to stop the horrid noise! The switch for the horn is in the center of the steering wheel - it may be tricky to get to this switch with an air bag unit in the steering wheel, something best left to a professional repair facility.
How do you repair 1999 Honda Civic Horn?
not having a horn
On the Steering...
red
The horn on a 2005 Honda Civic is located at the front of the engine compartment near the headlights. This position allows it to be projected out of the vehicle and heard better.
Middle of steering wheel
The horn sits in the middle of the steering wheel. There are horn symbols just left and right of the Honda badge. Press either one and the horn will sound.
The horn relay is mounted underneath the dashboard on the 1995 Honda Civic. The unit is mounted on the upper right of the cabin fuse box.
http://wiki.answers.com/Q/Where_is_the_horn_relay_on_a_1995_Honda_Civic"
behind the front bumper
The horn is located directly below the inner side of the passenger side headlight.
The horn button on a 1994 Honda Civic clip on to the steering wheel. If the clips did not break off the button should just snap right on.