FYI There is no backlit screen for the GBA, only front lit (i.e. the light source is infront of the screen). There is an afterburner kit which is fairly easy to install, though you need to cut the GBA shell. There is also the Halo Light which is like a screen replacement that has a light already built in:
http://gameboy.gamezone.com/gzreviews/r21400.htm

Anyhow if you want to go the Halo light route, I have a spare one if you are interested in.