Get a jump lead, clamp one end to the battery negative post and the other end to something solid on the engine, such as a lifting-out bracket. This is, obviously, only a test not a repair and should not be left in place, but it will prove or disprove an earth fault.
As John says, look for a thick cable from the gearbox to the body shell, also check the starter motor wiring connections.
bill22, April 2009
