1. Don't confuse positive and negative grounding with positive and negative switching. (In a DC circuit, when positive switching is used, a seized relay will lead to a dead short, and has been adbandoned by the engineering community.) All grounds are electrically the same, earth ground. (In this case, chassis ground)

2. Depending on which electronic theory you subscribe to, grounding can change. Electron theory (the one I agree with) says that electrons flow from ground towards the positive side of the battery. (observing "opposites attract" logic) The "hole theory" says that "holes" (absences of electrons) flow out of the positive side of the battery.