What you will need...
resistor 100 - 220 ohm
Electrical tape or shrink tube or Silicone 100%
--- I used liquid tape, very handy.
I had some slow RGB LEDs and a few ideas. I did some tests and narrowed it down to two ideas. I couldn't decide, so I used both.
The first LED pokes through the top lid. With the LED on and pointed at the ceiling in a dark room, you get a Red, Green, Blue light show on the ceiling. The second idea involved shinning the LED on crumpled tin foil.
holes to fit the parts. After the parts are soldered in place I covered the back with liquid tape. Next run wires from the battery/switch location to your LED area. I then partially assembled the phone and soldered the wires to the LEDs, battery and switch.
Before assembling the lid I lightly crumpled some tin foil an placed it to reflect the second LED. I held the foil inplace with... liquid tape!
Test your circut then reassemble the phone.
The LEDs were from Ebay. 12 LED + 12 Resistors for about $5
I got the slow change style. They also come in quick change. They are self contained and look like a normal LED.
If you use several together, they will stay in sinc most of the time.
A good place to find flashing items, batteries, LEDs
A good place for small Li-Po batteries
If using Li-Po batteries, make sure you use a Li-Po charger.