Interesting......you folks are getting a entirely different terminal design and I rather like the cables being bolted to the terminal.
This design, however, appears to have three potential ground resistance points.....all easy to service.
So, to get it out of the resistance path, I'd remove and wire brush all of the terminals and eyelets attached to the terminal clamp as well as the clamp to post interface and, if you can get to a few of the terminals at the other end of the cable, clean those as well.
Since the negative is off, freshen up the positive as well.
Beyond these suggestions, if not successful, you may have a internal problem with the bcm itself......something along the lines of a cold solder joint that would require replacement of the unit.
Since your car is not U.S. spec. I don't know the component location but once you find it, remove the multi wire connector and see if there is any corrosion visible (white powdery looking) on the male/female pins. Sometimes, the act of removing and reinstalling the connector will clean the connections and you can carry on.
Regards,
Rob