A second power take off and relays as shown above are the best way. You don't need to alter the fusible link, bulkhead connections, or bypass the amp gauge to properly add accessories.
The parts used back when were sufficient and served their purpose ( keep high voltage and fire potential outside the cabin ). Modern vehicles with alternators hot enough to weld with are wired the same way. Only small gauge wires and light duty switches to signal the relays are found inside those cabins .
The modern stereo head units we put inside the cabin will run on the wiring our OEM radios used. You need only to look at the wire and fuses supplied with your aftermarket head unit to verify this.
