Hello
There is no horn relay on this bike unless one has been added by a previous owner. The brown wire is the feed and comes from fuse F2 via the flash headlamp relay, The horn button earths the circuit so to test place a wire from the black wire on the back of the horn(s) to earth. If the horns work there is something wrong with the switch or the earth from the switch. If the horn still doesn’t work there is something wrong with the supply to the horn. Check all connectors, pull them apart and clean.
Here is Carl Allison’s wiring diagram click here beware sometimes Carl Allison gets the colours confused White (Bi) with Blue (Bl)
I haven’t finished drawing this model myself as no one has asked for it yet! See my own wiring diagrams click here