Our M3W's are -ve earth.
Most (if not all) modern vehicles are -ve earth because +ve earth leads to very rapid electrolytic corrosion.
I know this from experience because many years ago I fitted a Lucas 12V dc. car dynamo to a Vincent motorcycle combination that I owned at the time, and the dynamo had been polarised for +ve earth. The aluminium parts of the bike (of which there were many) very quickly grew a film of white powder and I was advised this was because it was +ve earth.
I re-polarised the dynamo and reversed the battery connections and the corrosion was no longer a problem.
