as my battery is in the boot, i just made sure that the neg has a nice fat earth cable, and just for good mesure, got some 8G amp wire and ran it to 3 other spots in the boot, like chassis rail, floor, back of backseat/parcel tray bottom.. then ran 1 or 2 extra to my head/block from the chassis at the front..

i was actually having dramas with the starter not earthing right.. so i connected it to the chassis seemed to work after that.. although the engine did have an earth, maybe it wasn't good enough? or covered in paint...