A small current passes through the metal you want to prevent rusting. However on a car it is difficult to make the current flow through the entire body because the car is an electrically isolated system.
2011-12-01 20:12:21 (y1750570)
If you try to pass a current through the body, it will find the easiest path to complete a circuit and won't flow throughout the entire body.This does work on any metals in soil or water such as bridges and marine vessels. Current goes through the hull or structure, and returns from ground (or water). This action then can protect submerged metal parts. That is hard to do on a car. That's why these systems are not popular.