The 20Amp fuse blows because a lot of current is flowing - the fuse saves the wires from catching fire.
A high current flows because of a short between +12V rail (wiring) and the chassis.
Take the battery out - put a fuse in (any fuse will do) and hunt around for the low low resistance - for 12V to blow a 20 Amp fuse the resistance will be less than 0.6 Ohms - I don't know vehicle electrics but would expect the current load to be in the order of 5 Amps total through the 20A fuse - that's a resistance of about 2.4 Ohms.
Set the multimeter up to show the low resistance that's causing the problem and bit by bit inspect the wiring - wriggle it about - and disconnect things. Find the rectifier and voltage regulator - remove them from circuit by disconnecting one terminal.
The problem will not be between the battery and the fuse - it will be after the fuse else the fuse would never blow.
The problem will not be a bad earth - a bad earth can only reduce or stop the flow of current.
You're looking for a dead short - could be the wiring but more probably a component device.
A micrometer is a measuring device for very small dimensions - you'll need a Multimeter - set it to Resistance (Ohms). Put one connection of the meter on the battery + wire and the other on battery minus (battery out of course) and leave them there as you hunt around.
A multimeter can be had for just a few pounds - Amazon - Maplin.
After you fix the fault replace the 20Amp fuse and refit the battery...
Good Luck...
August 2015
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