There are many reasons why this could happen.
The most common reasons are: shorted light socket, broken hot wire touching ground, bad ignition switch and not limited to a shorted battery.
What is the scenario when the fuse blows? Key off, on, run switch still off, on?
Accessories? Does it blow after a cause of a switch ....
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Remove the blown fuse and install a light bulb. I always use an old tail or running light socket and bulb. Connect negative and positive leads to the fuse socket. The light will illuminate due to the short.
Begin to unplug one load (light,cdi,etc) @ a time until the light goes out or dims to almost no light. When the light turns out then you have found the shorted component. If this does not work the wiring harness has a damaged wire.