Master shut off switch charging problem

You have to isolate the battery and alternator charge from the rest of the car.

If you have any wires attached to the alternator, like the normal black wire, it will not kill the car as the alternator will continue to provide power within that loop.

No way is a #8 cable large enough for the run from alternator to rear of car. You need a 4ga minimum, IMO. You are getting big voltage drop from the length of run and wire sizing. If you have some extra 8ga, make a jumper wire from alternator to switch at rear and see if the voltage increases. If so, you have a wire sizing issue.