I ground my trunk mounted battery to the body in the trunk. But I have a few extra grounds from the frame to the unibody and an extra one from the eng to the body. Working for a Dodge dealer for 24 years I would see some strange ground problems over the years like the one that causes the trans bearing to have problems or seeing sparks from a throttle cable because the body to eng ground is bad. So in my opinion if your not sure put another ground on it as you can not have to many grounds. Ron