It's best not to run the LED at their maximum current. Best to leave some headroom for voltage fluctuations.

I would suggest 20mA so (12-1.9-1.9-1.9)/0.02 = 315ohms (300ohms is the closest easy to get value).

If you calculate the resistor value at a current of 20mA the maximum LED current of 36mA will not be exceeded until the cars voltage is over 15 volts.

The top LED will not "see" 6.3 volts. Its positive lead will be at 6.3 volts and it's negative lead will be at 4.4 volts so it will only "see" 1.9 volts.

The current you chose will depend on how bright you want the LED to be, the lower the current the lower the brightness.

Rod.