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.