A lot of builds use the rectangular tail lights in place of the factory provided round ones for a period correct look. They have an inherent design problem of not passing light in the lower half due to the corner cube reflectors imbedded in the lens. I borrowed this picture from a post on the other forum by JimmyZ which illustrates the problem. The lights shown have been fitted with a LED flat board hence the dot pattern. At night there may seem to be adequate light coming around the reflector area but in the daylight it gets washed out.
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You can see the rectangular dark area in the lens where the corner cube reflector resides. I found two designs for these lights, one with LEDs fitted in the light, the other with one 1157 bulb for the upper half and the lower half blank. The LED one still has the dark area and the design with the bulb doesn't have a reflector. The back of the reflectorless light is open to dirt/water present inside the rear wheel opening. There are plans on the forums available for a rectangular box to cover the back side but this doesn't address the need for a reflector.

I'm helping on a build that the owner wants to use the rectangular lights and have two bulbs, one for stops light, the other for turn signal. Technology has advanced in the last few years where there are 1157 LED bulb replacements available that out preform the incandescent ones. I used the iBrightstar ones shown below which run about $17 a pair for initial testing. These bulbs have a lens on the tip for focusing some of the light and have red LEDs which will have a higher lumince coming through the red tail light lens. The LEDs around the cylinder need a reflector to project the light toward the tail light lens in order to get the full benefit of the available output.
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Here is a picture of the back side of the lens showing the corner cube reflectors. They are difficult to see due to the red color.
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I milled the corner cubes down so barely a witness of the pattern could be seen and flame polished the area.
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This picture is terrible but due to the amount of light being projected and being in the red spectrum, the cell phone couldn't do any better.
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With the reflector removed the lens easily transmits the light. For those with this tail light and want to improve the output, simply grind out the reflector. Polishing will improve the output but 90% of the gain comes from removing the reflector.