I'd first remove both battery cables and clean them and the posts well.

Then I'd slow charge the battery while everything is still disconnected.

While the battery is charging, I'd follow the neg. terminal cable/wire and remove/clean/replace if necessary each connection along the line.

Then do the same with the positive side.

I suspect you will find something simple is the problem.
Look for wires that may have chaffed against the body of the car somewhere. Especially related to components that move. For example...
I chased a short in my 745 for hours, hours and hours to no avail.
In the end it was simply the underhood light wire had chaffed against the hood. It looked fine from the outside, but the worn area was hidden from view. Very frustrating, but so obvious in the end.