The 750 supplies power to that fuse via the START button when it is not pressed. These switches are often bad now. The START button, when unpressed, has a light spring contact that connects that switch's Black wire the Black/Red (some are Black/Yellow) wire from the handlebar, and that plugs into the one going to the fuse inside the headlight. Most of them change wire color in the harness, but a few still come to the fuseholder's plug on the same color it left the headlight.
Over time, the lightly-closed (unpressed) switch contacts burn, especially if the headlight was changed to a halogen type. So, the 'quick fix' is to unplug the supply wire to the fuse, up in the headlight, and plug it into a Black wire there. The headlight will always be ON, but at least you will have one!
There is another fix: you can use a relay in the headlight, connecting the Black to the fuseholder's wire via the NC contacts, and then connect the relay coil to the START button so it cuts out when you press START. I make a kit to do just that, if you're interested.
If you decide instead to get a new righthand switch, be sure to use a headlight of sealed-beam type, and not a halogen bulb. Otherwise you will be right back here in about 2 years or so.
