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Carl's wiring diagrams are a terrific resource but they cannot be expected to show every model perfectly.I have learned to mix and match parts of different diagrams, Moto Guzzi tend to do that.I can't think why you would need larger than 15 with the following exceptions, I see a bunch of models around 2000 called for all 15 Amp, it's a safe size, all of the wiring will be protected.The starter relay fuse may have been upgraded to help with Startus InteruptusEarlier spine framed bikes used a 30 Amp between the Voltage regulator and battery but the diagram here shows no fuse, this would be easy to verify by unplugging the regulator double red and see if the loom side is alive with the fuse in.I wonder why your indicators went out, perhaps a short to chassis at the rear end where the loom goes through the fender.If it goes again tru replacing the fuse with an old headlight bulb (solder a couple of short wires on it) stuffed into the fuse socket.With that in place the bulb will supply the load and stay cold but if you jiggle the wires around and create a short the bulb will flash.