I am not entirely sure about the 2004, but in a 2001 Civic DX it is located behind the front bumper on the driver's side. You have to take the bumper off to get at it.
Middle of steering wheel
How do you repair 1999 Honda Civic Horn?
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.
its located under the left front headlight.
The horn is located directly below the inner side of the passenger side headlight.
inside the front grill
There are two and the are both behind the front bumper.
right in the middle of the steering wheel
Horn location for a 1991 Honda civic HBTo access the horn, you have to remove the front bumper. The horn is located on the passenger side of the car covered by the front bumper. You can see it by looking through the "bumper grill" near the passenger side front tire.
I had the same problem. . . I finally located the horn behind the passenger side front turn signal.
behind driver side bumper
Near the engine (inside the bonnet)