Ok.
1) companies that test LED's give a small pulse through them of 100 mA for 20 ms to see if they turn on which is far from what the thing will see.
2) High Intensity LED's are recommended to be run at 350mA Constant Currant.
3) the Vf (foward Voltage) is very important and will vary greatly because I highly doubt these guys are bin controlling their purchases... Anyways... its around 2.9 to 3.7 Volts per LED. and so depending if they are wired in series you could need upwards of 8 volts to run just two led.
4) by the same idea... if you LED's are in a parallel ciruict which they usually are.. you will now need 700mA to power two up..
So make sure your car can provide the required Juice... high power LED will not turn on at low currents.
Also make sure you can provide 350mA constant current and you did not jolt them with 1 Amp or so and burn them out... You can buy off the shelf 350mA current drivers from digikey.
While rare... you might have ESD'd the LED and it died... its like any circuit... give it a static charge and probability is you killed it... while rare on LED's it happens.
Also make sure you are keeping this cool. As stated before... LED's can hold up to 130C junction temp before complete failure... just turning it on if not properly heat sinked will get you those temps.
and the last one make sure the positive is to positive negative to negative... usually these star boards the LED come on, have 4 pads, and its easy to connect to both positives.
two pads are positive, two negative. you should be able to see the traces and figure it out.
hope this helps.