The receiver which measures the voltage pattern and decides if the rest of the voltage is allowed or not becomes the weak point. Like the encrypted key system used today, this could be buried deep inside the engine, but unclear why this is better than keys today.
In addition, if the driver should be able to manually recreate the voltage pattern by actually flicking the lights/wipers, there will be a relatively small number of voltage combinations which could be iterated through an automated device connected directly to the wires very quickly.