Well, since so many people asked, I will make a write up on DIY LED lighting.
what you need:solder gun
solder
LED's
CRD's or resistor's
wire
electrical tape
hot glue/gun
small dikes
small pliers
thin plastic or PCB boards
what you should have:hot knife
Dremel tool
hobby work station (like magnifying glass and alligator clips attached to a stand)
Get most all of this stuff at RADIO SHACK if you live in the USA.
I suggest buying LED's in bulk off eBay
First of all, you need to know a little about how electricity works. It's VERY basic and easy.
Specifically, 12v DC circuits and resistors and what not.
If you already have some basic knowledge or think you can hack it without it, then lets move on.
So, the most common confusion with LED's come from selecting the right resistor.
Here's a very simple site and calculator for choosing the right size for your application.
http://www.superbrightleds.com/led_info.htmThose first 3 blocks you enter the values from your LED.
I have found that ordering LED's in bulk off eBay is the cheapest way. I buy like 800+ LED's at a time, its so freakin cheap, im talking like $15 for hundreds of LED's.
However, you need to write down or save the email from your purchase because the LED data info does not come with the LED's i ordered off eBay!!
I had to dig up old emails and find the auctions again in order to find the voltage, mA, info for calculating the right resistor.
BTW, buy the brightest LEDs you can!! They are rated in MCD for brightness, I bought mostly 10,000mcd LED's but some are 6500mcd.
Now, if you can get the new CRD (current regulating diode) from Japan, then DO IT! I don't know if they are available in the USA yet, but if they are, BUY THEM!!
They are much better then resitors and ALOT easier to figure out. I use the 15mA CRD 153E. They are the most expensive part of my LED projects but much worth it. They are around 50cents a piece if you buy a few at a time, but you can get packs of 100 for about $30 so again, buy in bulk!
Ok, so first I will talk about basic LED wiring using the CRD's in case you can get them in the US now.

It's super freakin easy!!
1x 15mA CRD inline on the + side of a series of 3 LED's
Here's a 15mA CRD on the positive side of a 3mm LED

small eh!?
Yeah, now here is a resistor on the positive side of a 5mm LED

See how BIG a resitor is compared to a CRD?
Not to mention, for every # of LED's on your circuit, it requires a different resistor size.
For example, here is a 2 LED circuit, which requires a different resistor...

Ok, see with CRD's, you can use 1 CRD inline with 1 LED, or 2 LED's, or 3 LED's, after 4 LEDs the light dims a little (on a 12v circuit)
The SAME size CRD works for it all. No choosing or selecting. If you want to go over 3, like say 6, then just add 1 more CRD.
easy huh!?!?!
So bascially...
1-3 LED = 1 CRD
4-6 LED = 2 CRD
7-9 LED = 3 CRD
Get it? The SAME CRD size, (in this case 15mA) works for them all.
(btw they also make a 30mA CRD which acts like 2 15mA CRD's, get it?)
Ok, enough with CRD's. I HOPE you can get them in the US, i don't see why not. It is new technology, but should be available right?
Back to resistors since thats what is the "standard" in the US.
I'm not going to say "use this size resistor", but I will post pics and you can see the color code and decide for yourself.
Once you decide on a size of resistor, buy a but load of them.
I SUGGEST, choosing the size resistor for running 3 of whatever LED's you buy.
(oh yeah, this is another crap thing about using resistors, they vary depending on the LED size as well, not just the # of them in your circuit)
Another advantage of running CRD's is that IT DOESN'T MATTER what size LED's you run on your circuit, the SAME 15mA CRD handles it!!!! NO HEADACHE!!
Ok, sorry, back to gay resistors.
So when you buy your LED's, check the data info, voltage, etc, and then calculate using 3 of them in a circuit, then buy a bunch of the right sized resistor.
This is the most complicated part of LED circuitry, it gets confusing, but read up on it, its not that hard!!
btw, a resistor, or CRD, go on the + side of the LED circuit.
Yes, LED's have + and - sides unlike a light bulb.
If you peer inside the LED, you will see that one of the prongs usually shorter prong (+), is attached to the smaller stump inside the LED. Where as the longer prong (-) is attached to the bigger L shaped stump inside the LED.
Again, i refer to this picture, see the bigger stump inside is the negative side, and the resistors are attached to the positive side.

Now, get yourself some kind of power source, 9v is fine, just get a 9v battery, attached a plug to it so you have a positive and negative side, use it for testing your LED's, you will do this ALOT!!
Now make some circuit, try 3 in a row, whatever. Just perfect your soldering skills.
Ok now you know how to wire them up.
We will start with tail lights.
Now comes the HARDEST PART!
Cutting up your tail lamps! AARGH!
I forgot to take pics of mine, but i found these pics on the internets.
This is where the hot knife or Dremel comes in, I suggest the hot knife, it works WAY better, faster, cleaner, more accurate. A Dremel works too but it gets shit EVERYWHERE and destroys cutting discs.
There are basically 2 ways to open your tail lamps.
This is the suggested method. Simply cut out the BACK SIDE in the areas you need to get open so you can place your LED boards inside.


The other method is to separate the outer lens (red plastic cover) from the back plastic housing.
No pics because I don't suggest it. I did this for the first 2 tail lamps I did and fucked up the plastic, chipped the corners a bit. Kinda fucked up.
Dont even bother, just cut out the backs like above. It's easy to hot glue back into place when your done, you can also use some hard glues or JB Weld plastic, etc.
Ok, now that you have your tail lights opened up, you can visually inspect how you want to place your LED boards.
This is where your imagination and creativity is the limit. I have seen some people make plastic "back boards" and drill tiny holes and place lots of individual LED's int he board, but the most common and easiet way is to use the PCB boards you buy at hobby shops.
In all the pics you should see coming up, i used small 2"x3" PCB's. They have little copper contacts around each small hole on one side.
For my first time doing this, I just went with the easiest route and didnt modify my PCB's.
Here is how I laid out my LED's for each one of my tail lamps (i have 4 in all)

You probably notice there are both CRD's and resistors in that pic, thats because i ran out of CRD's and had to use resistors (they came with one of my LED orders)
The top and bottom row is for tail lamps.

The middle row is for brake lights. (yes, 3 different size LED's cause I was trying to make them brighter, still not enough)

And here is both on

Here is the back side. It looks complicated, but rather simple.
The 2 wires on the right are both positives, one is for brake and one is for tail circuits, the wire on left is the ground for ALL circuits.

You can see the little copper contacts you solder too on this side, notice its cleaner to put the resistors or CRDs on the other side.
Get creative and the more you do it, the more efficient you become in your circuit layouts and design. This is one of my intermediate designs, but havn't changed much since.
Its a very efficient diesng layout for my specific use/need.
Oh... before you button everything up, take it outside in the SUNLIGHT to check for brightness.
In the day/sun my brake lamp circuits are NOT bright enough. I re-modified my corner tail lamps a 2nd time and added the stock filament bulb back in, it sits behind the PCB board i made for each side, works perfect, I still get the LED effect for tail lamps, but for brakes, i still get the filament bulb birghtness.
Later when i have time, I will make it ALL LED for brake lights. I just need to use more. I obviously didnt use enough, i dont know what i was thinking.
I will triple the LEDs for my brake light circuits next time. My tail lamp circuits however are perfect!
BTW, i used all RED LED's, they simply look the best, so clean, so red. Using white works too but it washes out the red color to more of a pink/orange.
I suggest however, using all white LEDs for brake light circuits and all red LEDs for tail light circuits.
Here's the finished product with only tail lamps on.


Looks sooo good. (nevermind the wierdness of the far left corner lamp, in that pic it was still stock, notice its pinker in color.
Ok, here's another way of designing your boards.
front
notice 7 rows of 4 LED's

back
notice he chose a resistor for powering 4 LED's, using 1 resistor for each row
then you either put some tape on it or hot glue it up to protect your circuits from water
(if you think you may modify them in the future, dont bother glueing the backs, its a PITA!

I have since made 2 reverse light boards.
I modified the little PCB's, cut them down and attached them end to end making long skinny boards
Again, i was out of CRD's so i used resistors.
I ran 1 resistor for every 3 LED's
Its so freakin bright when I back up at night.



Here's a few other applications I played around with just for fun.
map light and door light






Toyota digital climate control



light ring around the ignition key ( i have since made a push button start in the key tumblers place)

Well, thats all for now, i will take more pics as I do more.
This weekend I will be fixing one of my corner tail lamps so i will take more pics.
Oh, I also have done my blinkers front and rear.
I'm going to add some behind each door mirror for a neat effect, maybe later put some under each exterior door handle.