Not to butt in here, but I thought I'd interject a little info on how to hook up LEDs...
LEDs have two important values: "If" and "Vf". "If" is the maximum safe current an LED will handle. "Vf" is the voltage the LED "drops" or is required to run. An inline resistor is necessary to limit the current to an LED to keep it from blowing. By playing with the resistor value, you can achieve maximum rated brightness or lower.
To figure out the value of the resistor needed, just use the formula (basically just Ohm's law):
(14.4 - Vf) / If
I used 14.4v as this is typically the highest voltage supply the car will deliver.
For the RadioShack LED part# 276-316 (2600mcd blue LED), the "Vf" and "If" specs are 3.7v and 20mA (.020 amps), respectively. So, apply the formula:
(14.4 - 3.7) / .020 = 535 ohms. Since this is not a standard size resistor, use the next highest value (560 ohms).
If you want to use two (or more) in series, use the same formula, except subtract all "Vf" values from the supply voltage. You can use as many LEDs in series as you want until the sum of their "Vf" values exceeds the supply voltage. At this point, they will never light up. For the RadioShack LEDs above, I'd go with no more than 3 in series:
(14.4 - 3.7 - 3.7 - 3.7) / .020 = 165 ohms. A couple of 330 ohm resistors in parallel will make this value.
If you want dimmer LED(s), reduce the "If" value in the formulas above by about 5-10%. So, for a 20mA LED, try using 16-18mA for the "If" value.
Once you figure out the resistor value, you should calculate the resistor's power value just to be safe. 1/2 watt should be more than fine in almost all cases though. Whatever you calculate for power should be doubled for safety.
(14.4 - Vf ) x If = Power
So, for the two-LED example above, the power would be:
(14.4 - 3.7 - 3.7) x .020 = .14 watts
Double that for .28 watts (or a little above 1/4 watt). So either 1/4 watt or 1/2 watt would be perfect.
Sorry for the long-windedness. Just trying to be helpful...
Edit: Oh, I forgot to mention - the longer lead on an LED is the positive lead. If you hook it backward, it won't break, it just won't light up.