Think about this,if the motor ran but blew the fuse while backing under load,replaced fuse backed again and blew fuse.something that you did caused the short to blow the fuse. If you look at a motor and trans under load the whole unit twists,if you put it in reverse it twists the opposite direction.worn mounts and such can exaggerate the twist. look for a spot on the harness,near engine or trans or even the driveshaft that is close to sharp metal.overtime spots can rub thru on the harness. harder to find would be 2 wires rub topgether. wiggle test as many harnesses you can see,then replace the fuse again. if you can get it started then watch the engine while someone backs it up under load (brake on).