So a Faraday cage will protect devices from an EMP... Theoretically, can you run a ground strap to your car body and use the metal body panels / structure as a faraday cage for all the electronic goodies inside?
In other words, use the body of the vehicle itself as the faraday cage to protect the ECM, wiring, electronics, etc.
Don't think it would work, Faraday cages are a closed and sealed container and tested by placing a radio inside. If the radio can receive a signal the cage is ineffective. The same test can be done using a phone. If you can call the phone and it rings, that's a problem. Same in a car, if the radio will work, the car isn't protected