I am installing a am/fm casette in a 51 ford that is 6 volt operation. The stereo is 12 volt operation. I am mounting a 12 volt battery in the trunk and grounding it to the car chassis. This will be the power source for the stereo. The stereo shows a ground on one side of the speakers going to chassis ground. There is also a wire out of the stereo labelled earth ground. I am wondering first of all if the earth ground is necessary and if so where would it connect to. Also, am I going to have a ground loop between the 12 volt battery and the 6 volt battery? Not sure about all this. I thought it would be simple but started looking at it and just not sure now. Any suggestions? Thanks you.