How about this "old-school" approach...no high-tech semiconductors required! 
Place the xmas flasher bulb in series with a 12V AC-coil relay. I figure a xmas bulb drops about 2V when "on." So the 14V Accessory AC would be split 12V AC to the relay, and 2V AC to the flasher bulb and the relay would turn on when the flasher turns on. Then the SPDT contacts of the relay would alternate power to the 154 bulbs.
OK, there are some details such as managing the voltage split between the relay coil and the bulb...but that could be fine-tuned by using one or more existing "regular" bulbs placed in series or parallel with the relay coil.
