Blowing fuse when put in reverse

Good job tracing the short. Sometimes their hidden in the harness and corrosion has set in due to moisture infiltration and sometimes someone just bungles something to cause it . Physical inspection is where to start as you have done ( good job ) . Ohm & volt meters , continuity checkers and short finders are great and often needed but there no substitute for a good eye to pick out damage , melted insulation and other calamities associated with wiring harnesses.
 
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