Two ~2 1/2" long screws located diagonally (catty-corner) across the fuse box retain the fuse box and firewall bulkhead connector to the firewall. The screws have 5.5 mm hexagonal heads, but a 7/32" hex head socket/nut driver will work. Once the two screws are removed, there are two plastic clips (directly below the screws) that must be pried outward, gently and simultaneously, to release the fuse box from the bulkhead connector. The fuse box can then be separated and rotated aside for inspection. The bulkhead connector will remain loose in the firewall, held in place by the engine compartment harness.
Once you have determined the cause of the short, to gain better access to the fuse box wiring for repair, disconnect the battery ground cable then separate the engine compartment harness from the bulkhead connector using a 10 mm socket on the single bolt protruding from the center of the bulkhead connector on the engine compartment side. Unscrewing the bolt will force the harness and connector apart. The inside cab harness can then be pulled away from the firewall and down toward the floorboard.
When reassembling, take extra care to ensure that no wires become pinched. Don't reconnect the battery ground cable until all harnesses are reconnected and properly anchored.