Well it sounds like you never heard of Kirchhoff's current law and the current divider rule. You can’t wire circuit protection devices in parallel without increasing the total circuit protection for any part of both paths. Even with circuit protection on your heavy gauge shunt wire it exposes all the factory underdash 12ga unfused wiring and components to more current in the event of a short than they can handle safely. Does nothing to address the factory loads still being drawn thought the bulkhead Packards, weakest link in the entire system. Someone should demonstrate how that would apply to this particular charging system in a video for those who don’t, or refuse to, understand.
“Kirchhoff's current law (KCL) states that the algebraic sum of the currents entering and leaving an area, system, or junction is zero. In other words, the sum of the currents entering an area, system, or junction must equal the sum of the currents leaving the area, system, or junction.”