OK, here's how you do it.

You take the lens dimensions and create a PCB outline to fit using a PCB
CAD program.  You then populate the entire area with the pads required
for the LEDs and matching series resistors, and connect the pads with
tracks to make series multiples of four LEDs and a resistor per circuit
across common power bus tracks.  Remember to allow for fixings to fit
the final PCB into the lens or base, and allow a couple of big pads for
attaching the connection wires.  If you have multiple circuits then the
LEDs can be wired in clusters, possibly with diodes to allow individual
or multiple groups to be used from a common switch line.

Now print the finished PCB artwork onto a transparent or translucent
medium and expose a piece of photo sensitive PCB material in a suitable
UV exposure unit.  The PCB can then be developed in an alkaline
developer based on Sodium Hydroxide, then rinsed and etched in a hot
bath of Ferric Chloride with air being blown through it to cause
circulation and agitation of the etchant.  Once etched you can then hand
drill the several hundred 1mm holes required for the components with a
small hand-held drill and then crop the PCB to size and shape before
mounting the resistors on the back, cropping the leads at the front and
then populating the entire front surface with high output (635nm?) LEDs.
If using superflux these are pretty good at self levelling, but 5mm LEDs
are best having their anodes soldered first then hand levelled
individually before having the cathodes soldered.  The choice of
resistor value can be calculated as 13.8 - (4*LED-Vf) / LED-I (20mA for
5mm or 50mA for superflux).  You can calculate the appropriate power
rating for the resistors by multiplying the voltage dropped across them
by the current and rounding up to the nearest power rating.

Now simply pop into your lens and bingo, there you go.

["Followup-To:" header set to sci.electronics.misc.]

a task like this would be good way to gain skills.

start with some perfboard (circuit board with lits of holes and no
cobnecteions made between them) figure out your LED layout stick the leds in
then join them up by the bits that stick out the back

by the time you've done the few thousand solder joins your design wants
you'll be an expert. :)

Personally, i would try diffusing the existing replacements by roughening
the surface and placing a thin layer of milky plastic in front of the
LED's.  A PCB full of LED's would look like a bunch of dots too.

I understand this.  I'm not looking for legal advice.

There's nothing wrong with the sealed beam construction itself, but the
classic DOT headlamps *suck*. Installing quality OEM E-code headlamps on
my European car was the best thing I ever did to it. People who ride
with me often comment at how good the headlights are and they're usually
shocked when I flip on the high beams. The original sealed beams were
downright dangerous, I was overdriving my headlights going 5 under the
speed limit on a dark highway, yet they still produced more glare to
oncoming traffic than what I have now.

That is not correct. 49CFR564 contains the specifications for replaceable
headlamp "light sources" (bulbs). The actual Federal requirements for car
lights, contained in 49CFR571.108, actually state the opposite of your
faulty understanding: The design and performance characteristics of every
light source are mandatory, and cannot be selectively modified to suit the
whim of whoever's doing the modification. Any modification -- different
power rating, different filament configuration, replacement of filaments
with arcs or arcs with filaments (or arcs or filaments with LEDs, etc.) --
means it is a different light source, not legally interchangeable.

Furthermore, 49CFR564 contains the specifications only for forward
illumination light sources ("bulbs"). Signalling light sources are not
contained in 49CFR564. The exact legal reason behind the illegality of
"LED bulbs" is a little tricky to follow: When such bulbs are installed,
they spoil the compliance of lighting devices designed to use a filament
bulb of one sort or another, and manufacturer-certified as compliant with
FMVSS108 when using that filament bulb. So it's not the bulb itself that's
illegal to sell, buy or install, it's what happens to the performance of
the vehicle lighting device as a result of installing the "LED bulb".

In actual fact, "LED bulbs" do not produce anywhere near the same amount
of light as a filament-type signalling bulb, nor do they produce anywhere
near the same (Lambertian) distribution of light...even those clever ones
with sideward-facing LED emitters in addition to the rearward-facing
items. Installing "LED bulbs" in bulb-type car lamps doesn't just spoil
their compliance with the applicable safety standards, it spoils their
performance. The light is too dim, the illuminated area is too small, the
intensity ratio between bright (brake or turn) and dim (tail or park) is
improper, and the vertical and horizontal angles of visibility are much
too small. All of this adds up to grossly reduced vehicle conspicuity and
safety.

DS

If I do come up with some sort of raw circuit board with hundreds of tiny
solder joints on the rear, I was going to cover it in a layer of silicone or
some sort of nonconductive insulator to help keep moisture out, although
these tail lights seal pretty tightly against weather and dust.

We shall see what happens.

Make sure you go for superbrites if you are replacing the lamps.
"Ordinary" red LEDs are in the 10-30 mCd range. You should be shopping
in the 500+ mCd range. If you get the 10 Cd ones you may end up with
something that will blind other motorists at night, but you can always
drop the current a little. Each LED will draw about 30-70 mA at around
2V.

You will probably end up wiring them in series-parallel, and you will
find in that case that some LEDs are a little more "greedy" than
others, and will draw all of the current off, so if you plan to use a
series-parallel arrangement, buy extras and then test all of them to
match voltages.

If you don't want to be bothered with all of that, you can just use
more resistors.

Here's the full list.... http://www.robotics.com/surplus.html 

Just look at them, legal or not, they're dangerously dim, at least the
ones I've seen in use so far.

Because he has data that shows "LED Bulb" replacements do not put out
the same amount of light as the original incandescent bulb.  And they
never will.  If you want LED lights, you need to either buy a
preapproved lamp or actually spend time and money designing one.  It
must meet the requirements in 49CFR571.108, also known as FMVSS 108.

I've looked at several of these LED replacements.  They are dangerous -
putting out as little as 1/10th the light of a standard incandescent.
Also, they only put out light in one direction - straight back if you're
lucky.  Lamps designed to use incandescents use reflectors - light from
the LED bulbs rarely hits these reflectors and so it doesn't get
distributed properly.

Photometric testing.

Actually, the legal requirements in force in North America agree with me,
and not with you. If you believe otherwise, your understanding of the
applicable regulations is faulty.

DS (the legal requirements in force everywhere else in the world also
agree with me, but that's beside the point).