the entire car 'is' part of the electrical system, every circuit grounds somewhere to the body. the body is a better conductor than even the biggest battery cable and with no heat sink front to back. as long as the power and earth cables are properly sized and more importantly the earth's are properly grounded at every connection they are fine. yes some ignition/fuel injection boxes are required to ground back to the battery but that has no bearing on slow cranking when hot. :thumbsup:
neil.