You ask why?
Why go through all this trouble w/incandescent bulbs?
Trouble? What trouble? I consider this a labor of love. Hey, chocolate and vanilla, my friend. Some folks like one but not the other, and some don't want to make the switch to LED as it's 'not period-correct.' Granted, the E9 Check Relay logic will never be baffled by substituting LED for Incandescent lamps (like my E28) - there is no Check Relay! But these cars have reflectors and lenses that are designed to work with a point source light located at the focus of the parabola, that's how they work.
And by 'work' I mean many things, I suppose it's the engineer in me. the correct color, Dim or Bright, On and Off at the right time, located at the extreme left and right of the bodywork, at the rear of the car, and (most important)
shining directly into the eyes of following drivers... no matter if the driver is directly behind you in exactly the right location, or maybe off to one side, or up higher above the pavement.
Lights are a safety item, a legal requirement, and a critical communication device, but not a fashion accessory. Converting to LED so one can brag that they have updated LED tail lights should not be their only motivation. NOT SAYING this is your motivation, but this has been for others.
By working with the existing design -- a parabolic reflector that collimates a light source into a beam, and a lens specifically designed to make this beam visible under widely varying conditions and viewable from a wide range of locations -- I am staying within the stock parameters of the design and the manufacturer's specification.
The fact that I have redesigned them to create a larger, brighter, and more reliable light is my contribution to keeping you safe and the car intact under an onslaught of idiotic people barely qualified to drive larger and heavier vehicles.
OK, you aren't the first or only; let us know
how they work after you do the installation (see above for a definition of 'work').
It would be useful to see a side by side comparison on the car, one light updated with those LED panels and one stock with new incandescent lamps. Take pictures viewed on axis, then off axis in all directions at five or ten degree increments up to about 45 degrees left/right, and 25 degrees above horizontal.
After all, you don't expect anyone to crouch down while driving behind you, with their nose 24 inches off the pavement, when you apply the brakes. If they are sitting several feet higher, I suppose it's still important they know when you are stopping, right?
I guess this covers one reason I go to 'all this trouble.' There are others.
Thanks for keeping the conversation going, and I look forward to seeing pictures of your results with those lights. I wish you every success.