I'll apologise now for the lack of detailed pictures as I only decided to make this instructable a month after the mod. If you're into hacking and making things your own then you'll be golden.
Step 1: Gathering Materials
As well as the toys you will need some basic soldering materials and LEDs. A glue gun will also be handy. I think that's it....
Step 2: Performing Surgery
Once you have successfully removed the innards from the toy either stitch it back up or glue it back together so to save the poor fellow.
Step 3: Figuring Out What Goes Where
Firstly I had to figure out how to operate the sound chip without having any external wires showing. I decided to make a push button switch which used an existing part of the Lego R2. This switch is sprung loaded so it only presses the pressure pad on the board once. Similar to when you squeeze R2's tummy to make the noise.
One you're happy with the switch you can add the LEDs. Basically I made up some led looms and figured out where to solder them so they came on when R2 spoke. Trial and error with this one I'm afraid but you can't hurt anything by touching wires onto points. It will either work or it won't lol!
Now the LEDs are sorted you need to get it all glued in and wires tidy. The LEDs sit nicely behind the Lego pieces so this built was crying out to be done to be honest. Everything fit perfectly without too much disturbance.
Step 4: Sit Back and Enjoy What You Have Created
Your Lego R2D2 should now speak and light up. Check out the video to see him in action.
Thanks for looking and enjoy making R2 come to life.