have the hatchback model with the AGM battery and have noted that chargers should have their Negative lead connected to the point on the body adjacent to the battery not the negative post on the battery. I've noticed that the heavy cable appears to be held by a large plastic nut to the body negative. Is it really a plastic nut or a slip-on cover over a proper metal nut? Have tried pulling on it but it doesn't seem to want to move, and the thought of taking a spanner to a plastic nut leaves