..the black wire originates at your fuse holder, where it goes up behind the front cowling to a 6-pin connector, and to your flasher relay as well..
..you will need a multi-meter capable of reading resistance (ohms)..
..disconnect the 6-pin connector, and read from the black wire to any metal (un-painted) surface...
..if you find continuity on either side of the connector's black wire, to "ground", then that narrows down the process of elimination..
..you might have a difficult time fonding a shorted wire, as it might take the vibration of riding to cause it to arc..
..but you can trace the wiring and look for rubbed insulation, which is a likely culprit...