The earth between the battery and the Engine ! The earth goes via the body first ! poor design really !
I put an anolouge volt meter between the earth terminal of the battery and the starter housing to test, now technically there should be no voltage reading if the earth is good, but if the earth is poor then the voltage looks else where and will travel via the voltmeter ! well at cranking I got 3.5 volts !!!!! anything over .3 of a volt is bad ! I then put a scope on the battery and the batt voltage was dropping too 7.5 volts at cranking ! no wonder faults were flagging up all over the place ! including "HDC disabled", "engine reduced power" etc etc plus the wipers would often do a single sweep at cranking and often just trigger off on there own even in hot sunshine !
I then fitted a substantial earth cable ( starter motor cable ) onto the neg batt terminal direct to a gearbox bolt by the starter. Since then no faults flagged ( even at -6 ) spins over lot lot faster even on a cold day and drives brilliant and the wipers have never done a random sweep since ! Batt voltage now stays at 10.5 at cranking as well. The whole car is 110% better than its ever been.
It took me less than an hour to diagnose and fit new cable and test but I had asked a dealer to fit an earth a year earlier but they said it wasnt neccesary! I should of done the tests myself 2 years ago but refused to touch it after paying 26K of my hard earned cash ! There are some photos on the gallery of the new cabnle and the screen shot of the batt voltage drop at -1 degree celsius !