I wanted a high brakelight, to avoid people rear-ending me in the morning traffic, - but I also wanted it to be almost invisible when not used. I bought a LED type brakelight which, when liberated of its futuristic plastic cover, consisted of a single long piece of PCB with the LED's on. As a first prototype (possibly the one which will still be on the car in 5 years time :-) ) I fitted this PCB into a small plastic cable-rail, which I painted black.
This is no wider, than it can comfortably sit on the upper rubber-trim around the glass-separator between cabin and engine compartment, without being visible in the rearview mirror at all. It is
simply fitted using double-adhersive tape, - in two layers at the lower part, to get the angle right.
From outside, nothing can be seen, until you apply the brakes - where the LED's are in fact below the black area on the top of the rear hatch.
Small gallery here:
http://www.matrasport.dk/Cars/Murena/Conv/P1.9i-8v-ls/brklight/index.html/Lennart