I replied Dan personally, but I'll add it here, hoping it will help people.
One has to be careful about a few points, or it could be sad.  I have 
two comments on malcolm's solution.  And then my reply to Dan is 
included.

Cetin

>
>You actually are running about 14v in the car, and normal LED's have
                                ^^^^ quite true....
>about that 2.5v drop across them, so you want a resistor that will pass
>50ma at 11.5v.  If you round to 12v (after all, resistors are commonly
>only 10% accurate), this works out to 240 ohms -- 12 = (.050)x(240).

Nononononono....  14 = (.050) x (280)  Soooo, 280 ohms is the min. you need.
(quite possibly more).  AND, this is the TOTAL resistance (see calculations
below).

>Notice that by doing this though, you mostly heat up the resistor rather
>that produce light.  You might want to series a bunch of these (like 5)

If you choose this route, use at least 6, more like 7, or the instant
you get a glitch in your regulator you'll fry one of these 5 (5x2.5 = 12.5,
and 12.5 < 14 ).

and now, the calculations:

======================================================================

>2.6 V @ 50 mA is the max. it will take.  I'll base the calculations
>on 45mA just so that you do not burn the thing.
>
>I'll assume you're getting 14V out of the regulator during normal
>operation.
>
>total resistance ~ 14/.045 = 311 ohms.
>
>now how much of this is coming from the led? ( 2.6 / .050 = 52 ohms ). 
>
>soooo, you need a 311-52 = 259 ohms.  (this is minimum; I would go
>with a 300 ohm resistor to be on the safe side -- the led will be
>sligthly dimmer, but it'll last longer )
>
>one more thing:  what should the power rating of the resistor be?
>        I =  14/(300+52) = 0.038 , R= 300,  
>        power rating = I**2 * R = .038 * .038 * 300 = .475 watts
>
>In other words, you cannot use the cheapest resistors, which come
>at .25 watt rating.  Use .5 watt or 1.0 watt resistors.
>
Cetin Seren				Direct:	412-635-3479
Software Development Engineer         	Main:	412-772-6600
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