Thanks, I understand what you mean. Everything on the dodgy earth could easily earth itself through the new earth, which probably isn't thick enough.
Easier with a diagram but imagine a cluster of earths on a shared bolt that has corroded,if you run an earth even a large one to the rear lights for example then they'll work no problem but the small cross section earth will now also back feed up to that corroded bolt where it's likely the remaining circuits will have continuity between each other just not the chassis, the main lights, side lights, dash lights, blower motor,horn and radio etc can earth down the rear light earth not suitable for that much load and unfused, hot hot hot and fibre glass!Talk me through that, I'm not sure I understand?
Same principal as setting the choke cable on fire if you forget to fit the engine earth strap,starter motor tries to earth through the choke cable to the dashboard locking nut.
That's it, you've got it.