Here is some info from the Spaghetti Menders website, please excuse the way it is written. He tried to use some humor by using his dog Noodles as the author.
I read through that and am unimpressed as he's written some of the same faulty logic that can be found in this very thread. All current passes through the battery ONLY if you have no charging system. The car chassis can be a good conductor without being made of copper simply because it has an extremely large area compared to ANY wires in the car.
Keep in mind that cars have been designed using chassis grounds forever. The only exceptions that I can think of are computer sensors that have a ground wire back to the computer. This is necessary because the computer is modulating engine timing and fuel mixtures based on subtle (very small) voltage changes from the sensors. You can't do this if the sensor readings change every time you turn on the blower, headlights, brake lights, etc.
