The car-s body is connected to the car's ground (common) so injecting a voltage into the steel frame (body) will simply cause the 12v positive battery terminal to float to x-thousand volts + 12. Also, as you get to high frequencies, you'll need to account for the impedance of the steel's conductive route to the ECU. An answer to your question can not be simply calculated/quoted without much more information.
Is your goal to disable a vehicle? If it was easy, it would have been done - not an original idea. If you are after a different goal, let us know.
I've worked on few cars that had been struck by lighting. Each time the car ended up getting scrapped in the end, Many modules would either be dead or full of faults.
The engine controls survived on at least one I know for sure so I'm not sure if it actually shut the vehicle down while running.