If something like a large Rumcorf Coil were used, the car body would act as an antenna, and the rf would capacitively couple to the engine compartment, pulling long white sparks from the polar mass of the ignition coil's windings. But there would also be sparks in any gaps in the body, something which wasn't reported. So it could have been some kind of magnetic effect, which is hard to shield with normal techniques. An ignition coil itself can work as a magnetic transmitter if it's fed with biphasic pulses. Reflected pulses from the top load hit oncoming pulses, producing floating magnetic splatter flux. Two of these can be phased to focus the vibrating flux at a distance, or sweep it in that direction. Normal materials can't shield against it.
