you have quite the drop and it is possibly the bulkhead connectors that are corroded.
but eliminate one other thing, where did you measure the ground from?

the vr sensing line should be using the vr case for ground.
same for the alt, you should be using the alt case for ground and not the battery ground.

you can also test again, by bypassing the sensing line and using power from the battery and see if your voltages change again.
if it is the bulkhead, you will see drops inside the car still, but your battery and alt voltages will be the same.