Blows the 20a, but none of the 10a fuses?
According to that schematic, there's 12 things 'tween the 20a and the gang of 10a that could be shorting to ground.
1- Red power wire to the rectumfinder.
2- The rectumfinder itself..
3- Red power wire to the ignition switch.
4- The ignition switch itself.
5- Brown power wire after the ignition switch.
6- Brown power wire going to the regulator.
7- The regulator itself.
8- Green power wire from regulator to outer brush.
9- The outer brush itself.
10- The rotor.
11- Brown power wire going to the fuseblock.
12- The fuse block itself, at its power-in.
You've already tried unplugging the stator, eliminating the hot brush and rotor, #9 & #10.
Try unplugging the regulator, which would eliminate #7 & #8.
Next would be to unplug the rectumfinder, which would eliminate #2.
A shade tree mechanic would bridge the 20a fuse, and watch for smoke/fire/flame...