Chagrin:
I can't see in the first picture clearly due to the glare, but the pin below the "0" on the chip labeled "1360", the ZXLD1360, should be floating (soldered to a pad that goes nowhere) currently. Wire the bulb with +12V and GND, then connect to that pin through a resistor (~10K) and to a PWM output on the Arduino. Voila.
Good detective work, however the datasheet says that for PWM use, the ADJ pin should be driven from an open-collector or open-drain output.