Going on the readings that a 5V drop isn't a good thing for a ground circuit, I decided to snag the Volkswagen's ground cable but was denied. Its battery terminals aren't the same size. But I had a thought.
Why can't I just jump the car to itself? The ground should follow the path of least resistance correct? That is, afterall, how a voltage drop test works. Hooking up a separate cable from battery ground to the engine should function as a new route for the circuit. And that's what I did.
And the car started! FOUR TIMES. It was a little sluggish, but each time, it did engage the starter and turn the engine over. I couldn't get the ground cable securely where I wanted to (on the clutch bell housing) so I reverted to the front of the engine on a lower bolt.
At this point, I'm assuming my problems are being caused by the BATTERY-->ENGINEBLOCK ground cable. This unfortunately is part of the entire wiring harness. I need a replacement ASAP. What can I do?