The replacement fuse should be 30A.
And your voltage hint is correct for voltage testing , but the thing you really want to do after the fuse/wiring repair is to take an amp draw reading on the red feed . It is common to have worn brushes/bearing in the motor and that
will show as a high draw [ which just makes the burn wire/fuse problem just return in a matter of time , along with burning out the regulator [ very expensive].
The original wiring and fuse were not sufficient for fan load longevity, and the new set-up will hold a normal spec. load, but not if the fan motor draw is beyond spec...
I have also seen some home-made mods that use the red to trigger a high Amp rated relay with direct/fused bat feed to solve the high draw from the fuse box.. the fuse [ #12] can then be brought down to 15A., as this is now just primary relay feed, with the relay contacts carrying the high motor/reg load....
Simular to the relay type set-ups guys use when they rewire for higher output headlamps...