I got 10.3 amps across the maxi fuse with all fuses in and the ignition off. Think I'll start by checking the ignition switch. JF
That's a heavy duty short if your measurements are correct.
Doing the same test (across the maxi fuse socket), start pulling the fuses one at a time to see what circuit has the short. The meter should drop to virtually 0 when you pull the fuse on the circuit with the short.