I would build a cheap circuit board that uses an LM317 (or similar) chip to regulate voltage. IIRC the LM317 has a 1.5 volt *headroom*, meaning the voltage drop through it will be 1.5 volts.
I think a good idea would be to run them in *strings*. 3 in series would be 10.8 volts. So, maybe you could plan on using 3 strings consisting of 3 emitters each.
Of course you could spend a little more money and buy a driver that regulates *current*, which is the way an LED should be run. In that case you could use one driver set at 160ma of current and run all of the emitters in one string. To run them all in series would require a *boost* type driver.
If you run them with resistors the current won't be steady as the voltage in the vehicles charging system rises and drops. The resistors that are correct at 12 volts won't be correct for 13.5 volts, and vice versa.
Here's a site that will help with the el cheapo home made circuit.
http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/