Here's an update regarding the brightness of the LED strip I used. I procured some tint samples and took day and night shots through each of 5%, 20%, and 35%. I had to lay the tint on the outside of the window, so it's bubbly and the light will likely be (slightly) more visible through properly applied tint simply because of refraction and all that.
35%
20%
I'm not sure what happened to the color on this one, but it's definitely red in real life, without this yellowish tint.
5%
Obviously, 5% is very hard to see through during the day. I would say they're all passable at night, so unless you prefer it brighter, this light should work with most tint. I'll add, though, that the light is slightly brighter looking in person than in these pictures. I tried to capture it as best I could, but on a white car, getting the exposure of the lights right is tricky.
Regarding availability of these kits, I'm still working on the logistics of making them (the money part), and it will likely be a few weeks before I'm comfortable selling them. This will also give me time to refine the design and test a few more ideas, so hang tight if you are interested in buying one.