It's fairly simple, and I think I can describe it.....

• Green/Black is 12V power (from the fuse). The Green/Black wire to the handlebar switch connects to the multi-color circuit board (MCCB) in the Green/Black area on the board.
• When you push the Horn button, power is connected then to a Brown/White wire, which returns to the MCCCB at the Brown/White area. (You can use a test lamp to confirm that.)
• Then both the horns pick up power from the remaining Brown/White contact on the MCCB in the Brown/White area. One contact and 2 horns means this wire has to be a "Y-shape" (or 1 into 2). Sometimes these wires are indeed Black. The 2 other ends of the "1 into 2" wire are each connected to one terminal on each horn.
• The remaining horn terminal is for Return ("ground') and runs back to the solid Brown area of the MCCB.

Testing
1. A test lamp on Green or Green/Black should be ON when the ignition switch is ON. If not, check the fuse spanning the Green-to-Green/Black area.
2. A test lamp on Brown/White should be ON when the horn button is depressed. (Just because it's not on medication doesn't mean it's not depressed.)
3. A test lamp on the Y-wire should be ON when the horn button is depressed.
4. If 1-3 is all true, then you have an incomplete Return ("ground"). Both Return wires need to go back to the Brown area of the MCCB. Attachment to the frame will not suffice.

Hope this helps.