If you have a multimeter with an amperage hoop, then you should be scanning the wires by tracing them with the hoop to see where the current draw is coming from and if you have a good Sperry or Fluke, it should be quite accurate. Add up the amps to see first if they total more than 15. Then trouble shoot the highest read wire to see if there's a fixable issue. Tedious at best, but you may have to clean and wiggle just about every wire on the bike to find the source of the leakage/draw. You probably already know this, but: the rectifier is making a DC electric wave form out of AC from the alternator. The regulator is only modulating the voltage. The current draw, not the voltage is what is melting the filament of the fuse. Current is usually a function of the device drawing the current. Example, you may have a shorted field in the starter that is touching the case and there's going to be current drawn even tho your not energizing the starter. You may have to trace a lot of wires, of just a few, but you need to find the current (amps) that are melting the fuse.
