According to Ohm's Law:
$$R = \frac{V - V_f}{I}$$
$$R = \frac{~12V - 3.4V}{16mA}$$
$$R = \frac{8.6V}{16mA}$$
$$R = 537Ω$$
So for a 16mA draw, 3.4V Forward Voltage of the LED, and an Average of 12V, you need a ~537Ω (next value up is 560Ω 10%) for each LED in parallel. Otherwise, you are risking blowing your entire array. Using a single resistor for leds in parallel is bad.
$$P = (V - V_f) \times I$$
$$P = (~12V - 3.4V) \times 16mA$$
$$P = 8.6V \times 16mA$$
$$P = 0.137W$$
So a simple 1/4W resistor will work.
Keep in mind, Automotive Power is typically 12V, but ranges from 10V to 14V or higher. The 560Ω resistor is good for that range, keeping current between 14mA and 20mA depending on actual input voltage.
See LED matrix dimming: How to control the current in a 12V LED lamp for video lighting? for some more info