disconnect the rear tail light at the read tail light connector and reinstall a fuse and see if it blows the fuse.
let me know what you find
the short is in the connector to the tail light. first check for any corrosion in the light sockets and terminals. if no problems are found in the sockets or terminals then you will need to remove the wiring from the rear of the bike to inspect it for any shorts to ground. like cut wires rubbed thought insulation and or green corrosion.