Well, a clone of the Arduino Pro Mini can be had for probably about $1.50, a small processor board that is factory programmed to blink the on board LED. So no programming is required if all you need is the blink. The blink is also available on a pin of the board, that you can simply connect to another LED with the usual resistor. It has an on board voltage regulator that can step down 12V to run the processor and LED. So all you would need is the board, a resistor, and an LED. Pretty simple. It is off the shelf working with no user programming required.
Edit - actually it's possible to drive LEDs alternately with an output pin, so that suggests crossing grade flasher too...