3M 4947F should fit the bill. It's black, UV-resistant, and 49 mils (1.1mm) to dampen both vibration and be thick enough to wedge something between the glass and carbon housing in the event it needs to be removed. It's designed specifically for adhering items to glass and resists both paint and plasticizer. Carbon fiber and fiberglass resin—especially when molded smooth—has enough surface energy that adhesion and any possible off-gassing should be a non-issue.

I am already planning on including three felt patches, the edges can be seen from the rearview, but even on untinted glass they're nearly invisible. I'll look into making a cover of sorts to see how best to handle it. I want to avoid any hard surfaces near the glass though, rattles be the devil, ya see...
I was thinking about this, but I had a HELL of a time trying to get that center button off, I just gave up. And I was using interior panel tools etc. I do have the ability to bend and even bond an aluminum foot as you suggest, but it's not going to happen with the first design. It's on the radar though and I'll keep you guys posted.

The light is really bright, enough to bloom on a camera. It very very close to the same color and intensity as the OEM tail lights. I'll take a photo if you'd like.

4929, 4949, and 5952 are all considerations in the line of ~45 mil black VHB tapes. Do you think 1/4" would be sufficient or should I stick with 1/2"? 1/4 would be easier to remove if necessary.

4929 / 4949 / 5952 are all for bonding metal and plastic. 4947F is specifically for bonding various substrates to glass. While all of these should work, my money is on 4947F above all else. I have experience with that exact tape.

1/4" wide is more than enough. Overall weight is likely under a pound and most VHB has pull strengths in the 30+ PSI range. Even a 1/4" strip should take 10 PSI or more just to pull off. I'd rather fight 1/4" worth of VHB than 1/2".

Keep in mind this could get dust, bugs, haze, or other unknown variables behind the glass and require periodic cleaning. I could almost see a version with a long tail that tucks into the headliner and puts enough pressure on the housing against the glass to stay in place with only a felt lining. Cleaning would be vastly easier. It could even bolt to the antenna area. Just a thought.

okay cool.
i actually did this too but i added the aluminum foot and the headliner button goes through a hole in the aluminum to keep it stable and up there without the need for 3mtape. however due to the aluminum, the part that was supposed to be flush to the headliner is slightly lower then it now.
Also what light are you using, can i get a link.
my light is near invisible in direct sunlight through my tint.
at night it is very bright tho.

I am using the same light from the first thread I referenced, seen here. It is this light from www.superbrightleds.com

interested.

Ah perfect, that's exactly what I need. Unfortunately they don't make 4947F in 1/4 wide rolls. My supervisor was under the impression some vendors can cut custom widths... I'll make some calls and see if that's possible.

I'll work on a way to mount it with a bracket, seems people definitely want that option, but its going to be more expensive. Using tape is the quickest, cheapest way to mount a razor thin light like this. I guess it is possible for debris/etc to enter the housing, but with felt covering the areas where tape isn't (moreso than seen in my photo), accumulation should be minimal if any. Nearly every headliner I've pulled from several different cars revealed a pretty clean area behind it.

To boot, the top edge of the carbon flange actually butts against a piece of foam or something bonded to the roof structure/skin, so in combination with the felt it sort of acts as a poor-mans labrynth seal.

Depending on the volume you produce, fabricating a cutting jig for the tape wouldn't be difficult. It's added time and time is money. If you only crank out 20 units then a custom tape order probably wouldn't pan out. You'd have to do the math on length versus cost. Each housing is ~2' worth of tape. One roll of tape would last a while.

Tape is indeed the quickest and easiest. If the housing is sealed well enough to keep out dust/bugs/haze then I'm all for it. My BRZ windows seem to get hazy quite quickly despite not smoking and the car being over a year old. The plasticizer used in the interior panels must still be off-gassing.

Nice!

I'm absolutely 100% in if you start producing these.

mm. Me likey. Me dont likey the carbon fiber because $$$. If it was FRP or fiberglass thatd be a different story
..maybe more affordable then.

in for GB.

Just posting so I can follow about a GB.

I'll throw this question out there as no one has asked so far. Does the housing cover over any of the wire grids for the rear window de-fogger? Asking because if the LED should fail, an easy removal without damage to wire grids would be necessary.

If the housing doesn't contact any of the grids, would black RTV silicone sealer be an option?

I'm interested in a group buy. I'll keep an eye on the thread.