The positive is at the right side with the red cable going to the starter as you mention. I have to check where the black cable (from the positive) goes, it doesn't go to the body I can assure you that, it must be powering something else.
The ground or negative cable is easier to explain. I have a battery cut off switch installed in the left hand compartment (where the hood opening handle is), so that's why the ground cable doesn't go directly to the body. It goes from the battery to the cut off switch and then to gound.
Not only is this a 50 y/o car, but one that was messed around with in the 90s when it was converted to a CSi. Add to that the fact that I am in in Mexico, and there's probably less than 10 E9s in the country, so I have nothing close by to compare it with....and there are no mechanics that have specific knowledge of the E9.
I appreciate you pointing these things out so I can investigate and return to original whenever possible.
Same story with the water tank, I never realized it was incorrect or modified, now I have one more thing to fret about...
Enjoy the weekend!
Henry