As others have said, homemade (or home-modified) exterior lights are a dodgy game. It's possible to make them work, but it's a lot more "involved" than most people realize.
LED signalling lamps (brake, tail, turn...) are appearing on cars, and are
now almost universally used on trucks and buses, but it really is not a "retrofit" item in the sense of dropping in an "LED bulb" or putting some circuit boards with an array of emitters behind the lens and calling it good. Bulb-type brake, tail, parking, and signal lamps on your car rely on a point source of light (glowing filament) that radiates in a Lambertian pattern (more or less equally in all directions -- a sphere of light). The reflector and/or lens optics are designed to collect and distribute that light. LEDs generally give a very different kind of light distribution. Most of them don't produce light in an even sphere. Instead, they project a very narrow beam of light in one direction.
For any automotive lighting function, not only is it crucial that the intensity be within the proper limits through the entire relevant range of vertical and horizontal angles so as to provide a recognizeable and penetrating signal to observers at any angle to your vehicle (not just straight in front of or behind the signal), and not only must the intensity ratio between bright and dim modes be correct at all angles (for combination brake/tail or park/turn lamps), but the effective projected luminous lens area must not be reduced. EPLLA refers to the amount of lens area significantly lit up when the lighting device is active.
Look closely at the optics of one of the newer vehicles that has LED brake/tail lamps. You'll see optics totally different in configuration compared to those found in bulb-type devices. These special optics are necessary to coordinate the light from a large number of LEDs (relative to the overall size of the device) to get everything right in terms of brightness in both dim and bright mode, uniformity of brightness throughout the visibility angles required by law, ratio of intensity between "bright" and "dim" mode, EPLLA, etc. These kinds of optics are generally not something that can easily be made at home.
Then there's the issue of thermal management. Remember, LED output is highly dependent on emitter temperature. The range of output is huge, and you have to have some pretty sophisticated variable-resistive and/or PWM control circuitry to make sure the lights perform correctly/safely whether the emitters are really cold, really hot, or anywhere in between.
That said, if you are determined to have LED lamps, you may want to get hold of some Osram Joule emitters. These are Lambertian LEDs intended to be used in more traditional optical packages (central emitter w/bowl reflector with or without lens optics). These are used in the '06+ Mercury Mountaineer, the new '08 Ford Taurus/Mercury Sable, and the new '08 Chevrolet Malibu LTZ. Do a web search on "Osram Joule" and you'll find it. They are not meant or designed as direct replacements for bulbs, but they might be a starting place.
For another data point: I can think of exactly
one individual who retrofits LEDs into existing signalling devices and turns out a consistently good and durable result that meets photometric requirements. Take a look at
his prices. They're high not because he's profiteering, but because the work he does takes a great deal of time and uses costly materials and parts.
