My Stebel was installed at a wrench party few months ago and has worked OK til now.
When horn button is pressed, I can hear relay clicking and headlight dims out but no noise out of horn.
Haven't "honked" it much and have not run through any water or mud.
Any ideas?
The fact that the relay clicks and the headlight dims indicates that the horn is pulling a lot of juice. It sounds to me as if something is interferring with the armature rotation. Those horns work by spinning a little electric motor that has a vane fan attached to one end.
I had one do the same thing and a pebble had worked it's way through the air passages and jammed the armature.
If this is what's happened, it's either dismantle and repair, or replace.