I think you can do it by directly grounding the negative battery terminal. It is the same point as the green wire from the regulator. According to the drawing, the battery is only indirectly grounded through the starter. If you put a volt meter from the battery negative to ground is should read zero volts. Then switch to ohms. If you read any low value of resistance above zero, then that is where your problem is coming from.
Instead of changing the wiring, I would recommend that you find the existing bad ground connection(s) and fix it(them).