If engine speed PID on the CAN id is more than 0, then the arduino is to output a continuous signal either negative or positive to drive an external relay.
Using a simple comparator is much simpler, just measure battery voltage when ignition is on and if over 13 the engine is running. This varies some from car to car. This may give you an idea: Replace the 10K with a Blue or White LED, any color would work. The LED has a very sharp knee and will possibly be more stable then a zener.