fuses are blown due to more amps crossing the fuse then the fuse is designed to hold. In this case 15 amps. I would check the wiring with a multimeter and get the amp readings to get an idea what power is coming off the switch. From there back track the wiring and find out where the faulty component is.

What wires come into that fuse? what components are on those wires? Check all those components as well as whats behind those components.
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Charles Gallant