Re: 20 amp fuse keeps blowing
If this power fuse blows only when you turn the key to the START position, remove the 3/8" terminal wires from the solenoid, then retry (with new fuse installed) turning the key to that START position again. If the fuse still blows, there is a short in the wire leading from the "S" terminal of the ignition switch to the 3/8" nut solenoid terminal.
If it does not blow, then either the solenoid or the starter is shorted out.
If the fuse blows simply when you turn the key to the ON position, some component or wire is shorted out that leads from (connects to) the "A" (accessory) terminal of the ignition switch. In this case, finding the problem would consist of a matter of elimination...... disconnecting items until the fuse no longer blows.