Just wanted to close out on this issue, which has now been resolved.

My mechanic is a champ - he never gives up.

He had a hunch that the main wiring loom that connects the rear of the car to the front was somehow faulty. However to prove that, he did have to remove half the interior of the car to get access to the wiring loom.

Turns out he was right - one of the main wires in the wiring loom is faulty. But instead of replacing it with a new loom, he replaced the section that was faulty. That's because the genuine replacement wiring loom from BMW would take weeks to get in and would cost an absolute fortune.

All is good again.

According to my mechanic, it looks like the vehicle has had a rear-ender and the accident repairs did not include replacing the loom.

The problem isn't the safety terminal on the battery positive post. The faulty wire was a section of wiring that is not visible. Must have been a big crash previously! :eyecrazy Hope this car is not a dud.

Anyway, $380 later, the vehicle is back on the road, and I have a spare battery from my misdiagnosis. :rolleyes