On a few of the HD lineup, there have been some problems with blowing fuses in the lighting circuits for no apparent reason.
If you check where the wiring goes through the rear fender from under the seat area and follow it back, the wires have sometimes become disconnected from the wire loom there and are rubbing the rear tire. It was also a problem when the rear tire bottoms out from carrying a heavy load and rubs through the wiring harness and shorts out. Repair/replacement of the harness will cure this problem.
If you check where the wiring goes through the rear fender from under the seat area and follow it back, the wires have sometimes become disconnected from the wire loom there and are rubbing the rear tire. It was also a problem when the rear tire bottoms out from carrying a heavy load and rubs through the wiring harness and shorts out. Repair/replacement of the harness will cure this problem.