One has to understand the principle of CP to understand that the technique works by forcing a protective flow of electrons to the metal that needs protection. For this process to work, you need a complete electrical circuit to bring the electrons back. In the case of an outboard motor on a boat, the sea water completes the circuit. In the case of a bridge, the wet soil completes the circuit.

But in your car, the only way to complete the circuit on all the metal in your car is to drive into seawater or be buried in soil! There are various products on the market claiming to provide cathodic electrochemical protection to your car, just by injecting electrons into your metal work - but they don't work. Countries like Canada and the U.S.A. have actually got court orders to stop these products from being sold - simply because they don't work. In your car, there are lots of little nooks and crannies where dirt and/or water can collect. The rust happens not where the metal is dry, nor where the metal is wet - but at the interface between the wet and dry metal. So if you screwed a bunch of anodes right on the interface or one or two millimeters thereof, you would protect your car. But you would need thousands of these anodes over your car.