For those of you experimenting with replacing 12V bulbs with LEDs,
here is a quick lesson.

LEDs come with two ratings, a "Vf" (voltage drop forward) and a
current/amps rating ("If")

Most LEDs have Vf of around 2.5 volts (ultrabrites tend towards 3.5v)
and If of 0.02 A (20 milliamps), although there are some weird ones
out there so read the label.

LEDs are not like normal lightbulbs: if you over-drive (pass too much
current through) them a little bit they will get brighter, but if you
really overdrive them they will actually get dimmer.  It's one of
those weird quantum mechanical effects that boggles the mind, but is
true.

So basically, in a circuit you treat an LED as if it has a fixed
voltage drop and wire a resistor in-series to limit current.

The resistor value is calculated as
   R = (V - Vf)/If

Where V is the driving voltage (in a car, 12v), Vf is the LED's
voltage drop, and If is the LED's current (expressed in Amps, not
milliamps!).

So for the typical bright LED (3.5V, 20mA) in a 12V system you get

R = (12 - 3.5)/.02  <-- remember use Amps, not milliamps!
R = 8.5/.02
R = 425 ohms.

Feel free to adjust R up or down by 10% or 20% to tweak the
brightness as you see fit.

If you want more brightness, you can also wire a set of 4 3V LEDS in
series without a resistor at all.

Shameless plug:  a friend of mine makes light-up clothing and
sometimes I get to play in the factory :-)
   <http://enlighted.com>  (Warning: semi-worksafe link)