If you study the factory wiring diagrams you'll be surprised to learn that many of the accessories this car came with are not fused, other than the fusible link outside the bulkhead. You have only 2 fuses hot at all times, 2 fuses that are hot in run, 1 for dash lights. Most of these are shared by multiple circuits, reverse lamps share radio fuse for example.
So if you parallel another electrical system for all these addons you will need a higher output charging system. That one charging system has capability to send more current through all the OEM wiring too. 60 to 80 amps could travel a wire, through a switch, designed/rated for 42 amps max. Look closely at its condition, consider upgrades. Good luck with it.
