Good Method to Add Wiring For Add-Ons
Here's the threads with diagrams and photos for a couple of safe battery in trunk arrangements.
questions about battery disconnect wiring
Trunk Mount Battery - Wire Routing
Adding electric loads, especially equipment that will be running during start and while idling can cause problems with the original wiring strategy.
The original system only drew on the battery for starting and to keep things running for brief periods such as when at a stop light with wipers, heater, and lights on.
This
post suggests two approaches when adding electric fuel pumps and fans.