According to this page in the SRAM spare parts catalog, clear plastic piece covering the gear indicator is called the lens. I wanted to put tiny LEDs under the lens to make the indicator visible at night.
In June 2013, I made this proof-of-concept. After that it stopped working, and then I got a new bike.
I chose sub-mini LEDs because of tight clearance inside the lens. Red was the natural color choice for three reasons: first, red light has the least effect on dark adaption; second, it matches the red gear indicator arm; and third, red is the most common and cheapest LED color.
At Fry's Electronics, I found I found some Linrose B4307T1s which are very small (1.65mm square) through-hole LED modules. Wires from the lights would route through braided PET expandable sleeving, also from Fry's, to a small battery pack.
I could fit two LEDs in the shifter lens, one on top and one on the bottom. I drilled small holes for the LED leads to snake through:
The leads were thin and fragile. Soldering long wires to them acted as lever arms made the even easier to break. I cast everything in place with epoxy, then removed the excess with a Dremel tool.
A lot of wires broke from metal fatigue. When I attached everything with epoxy, I broke wires when Dremmelling away the excess.
Epoxy's viscosity and stickiness made it impossible to control during application, and I got glue all over the lens.
I had started by painting the outside top and bottom of the lens with white acrylic paint to prevent light from escaping. This was a very bad idea because glue sticks to the paint, instead the plastic underneath, and then the paint comes off.
Here's the graveyard of wire harnesses:
I stopped trying after four failures.
The lens is thoroughly scratched from Dremelling, has marks of white paint, and poorly-aligned holes.
In September 2015 I got a new bike which doesn't have gear indicators. Problem solved.