This what you're talking about is not a safe modification. LED signalling lamps (brake, tail, turn...) are appearing on cars, and are widely used on trucks, but it really is not a "retrofit" item in the sense of dropping in an "LED bulb". The "LED bulbs" that are available all over the internet are dangerous junk. The brake, tail, parking, and signal lamps of your car rely on a point source of light (glowing filament) that radiates more or less equally in all directions -- a sphere of light -- collecting and distributing that light with optics in the lens and/or reflector. An LED is a vastly different *kind* of light source. Unlike a glowing filament, it does not produce light in an even sphere. Instead, it projects a very narrow beam of light in ONE direction. That's why these so-called "LED retrofits" are unsafe; there's no way you can get enough light through a wide enough angle (horizontally and vertically) to create a safe and legally-compliant lamp. This applies even to the fancier "LED bulbs" that have side-facing as well as rear-facing emitters. The problem is not with any marketer's particular implementation, the problem is with the concept, which does not (cannot) work.
There are other considerations, too -- 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 only must the intensity ratio between bright and dim modes be correct (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. With "LED bulbs" installed in lamps meant for filament bulbs, you tend to get a little dot of light with the rest of the lens almost completely unlit. So not only is the visibly lit area dimmer, it's also smaller. Safety? Not so much!
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 not something you can kludge in your garage, let alone achieve with these unsafe "LED bulb retrofits".
Lighting devices meant to take bulbs need to use bulbs. Retrofitment _can_ be done, but not with "LED bulbs", nor with LED "panels" sold by some vendors to fit various classic cars. Safe/effective homemade LED lamps are possible but not simple to construct. See threads here for two examples of homemade LED light projects with the right amount of thought, effort, understanding, and technique behind them to be probably safe:
Thread 1
Thread 2
For those who lack the tools, expertise, and equipment to make their own safe and effective LED lights, and who are wise enough to avoid the fast-talking profiteers—the vendors who offer "LED retrofits" for classic cars—but who want LED lights, you can sometimes get clever with ready-made truck/bus LED lamps placed inside the lenses of your car's original lights. You have to be careful to get the placement right; they need to be upright, facing straight, without any slant, tilt, or inward or outward rotation (the exception is the units made specifically for postal trucks with a 7° forward tilt to the rear surface where the taillamps are mounted). If the units you pick have a "TOP" marking, it must be at the (duh) top. My favorites are these, in clear-lens variety when available; their performance is excellent and they aren't expensive:
http://www.levineautoparts.com/vali6ovledla.html
http://www.levineautoparts.com/sttutareled4.html
http://www.levineautoparts.com/truckliestt.html
