I've just looked at the wiring diagram and Bryan is right; when the ignition is on there should be 12v on the black wire and the horn button supplies the earth, so the problem is likely in the button, not an earth (frame) connection. My mistake. So if you earth the light green wire inside the headlight shell connection and the horn works, the fault will be back toward the handlebar switch, probably the wire has broken off the contact that the button earths. It can be dismantled and soldered, but it will test your dexterity and patience, big time! Dismantle it in a large, uncluttered area if you can, because bits and pieces may go flying and be hard to retrieve if the area is cluttered. Might help if you can put a large plastic bag over the switch as you dismantle, just in case. Once you have it apart, you'll be able to become familiar with how it goes back together, but it'll probably take several attempts. If you have a multimeter, you can see if the horn button is working by connecting the green wire to one lead and the other to the handlebar and checking using the ohms scale. That reminds me, the bars aren't painted are they? I think the horn earth is via the bars, so if they are'nt properly grounded, that could be your issue.
