that is not always the case. I remember we had a newer Nissan truck that we put a large stereo with multiple batteries and we thought we would do it right and go straight to the trucks frame. Later that we we could not figure out why the truck seemed dead, completely dead. After some close looking the frame was isolated from the car with bushings and I guess what ever coating they used on the bolts resisted current as well as we were only able to get 2v max out of the car now with the battery ground where it was.