Brian's correct, there is no difference between the ground and "voltage dump" point. They can share continuity, as they are meant to do. This is, by design, a shunt regulator. When a high voltage is seen across the circuit, it shunts to ground to protect the equipment.
Sure, you can pull that ground and you will still have lights, but the regulator will not function as designed.
My guess is that despite your claim that the wiring is fine, it's most likely a short somewhere in the aging harness, which is either shorting the bulb, or more likely: introducing a second V input into the circuit.
Either way, some time with a DMM will sort out the majority of your issues. I would not suggest wasting the time isolating the shunt ground from the regulator.