Hey wassup guys i got a question that maybe somebody can help me with...

Alright what i want to do is make an led that you could buy from radio shack or something and have it flash like one does from an alarm system but ONLY when the car is turned off...now i have been thinking about how to do this and the only thing i can think is use like a resistor so high that you have two power sources running to the light, one is a constant and one turns off whenever the car is turned off...im not sure if this would even work just a "sort of" theory...well any suggestions are appreciated.. thanks!