Just to see if I get this right: you want to modify the brake light of your car in a way that it starts to blink before it goes solid? Here in Germany this would be most probably illegal and for good reason. In case you would be involved in an accident that might be correlated to your invention you would be in deep shit. Maybe this is different in your country but you should consider this.
Anyway what you are creating will most likely not work reliably in a car. It may work initially but eventually it will start to fail in strange ways. The reason being that the power supply inside a car is "very dirty". You may want to read this http://www.littelfuse.com/data/en/Application_Notes/an9312.pdf before you actually put your contraption into a car.