I was also always sceptical about this too; and still am. But while I believe normal and common industrial cathodic protection (based on a sacrificial anode immersed in water) won't work on cars due to the lack of a water medium there is another form of electronic protection called anodic protection that appears to be soundly based scientifically and engineering wise and if a suitable medium coating is applied to the car's body both cathodic and anodic protection may well work. Some sellers of these devices do also claim to coat the card with a special medium, (but I wonder in this is just really rust preventing deodorised fish oil rather than a conductive medium)
What makes me wonder if anodic protection at least might be made to work on a car is a quote from the well regarded Bosch - SAE (Society of Automotive Engineers) Automotive Handbook (6th edition in this case but similar things are in earlier editions) ' ...another option [to cathodic protection which moves the potential so no anodic current flows or using a sacrificial anode] is to shift the potential of the threatened electrode into the passive range between Up and Ud. This is called anodic protection. The anodic currents, which flow in the passive range , are less than those in the active range by exponential powers of between 3 and 6, depending on the type of metal and the corrosive medium. The result is excellent protection for the metal. However, the potential should not exceed Ud as oxygen would be produced in this transpassive range, potentially leading to higher rates of oxidation."
Certainly some sellers of these electronic corrosion protection devices claim that as the use anodic protection the lack of a suitable medium isn't an issue. See
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'The anodic protection provides significant reduction of the formation of corrosion to painted and unpainted surfaces, proven by independent laboratory tests conducted in accordance with ASTM procedures for the testing of corrosion resistance in the presence of salt fog and extreme humidity. "