I'v been lurking for ages here, i'm also doing an electronics degree and tinker a lot. For ages i'v been wanting to try the frontlight mod so i finally got a cheap broken SP on ebay and spent this afternoon doing the switch. Results are ok (pics in a minute) Thought i'd share my experience though.
I was following the guide by Bibin and have some things to add.
1. DO NOT rip off the cable connected to the LED, de-solder it or otherwise you'll do what i did and break the damn trace. Leading to extremely tricky SMD soldering, i did it though!
2. As i broke it, i was able to view the led in all it's entirety and beside it is a surface mount 120 ohm resistor, labelled '12'. (There's no way it's 12 ohms as that would give 250ma current to the led meaning it wouldn't last long and cripple the gba run time.) So the led is supplied with 25ma current, something to bear in mind when modding as when you add in your own resistor it will be added to the 120 ohm already there.
Not a problem for me as i soldered directly to the led and as i already had wires hooked up to the 5V cartridge pins i used a 100ohm resistor giving my led (assuming typical white led voltage of 3V):
5V-3V/100 = 20ma
The actual contruction of the light bar was quite interesting as there is only 1 led that is aimed towards a diffuser bar which evenly (kind of) spreads the light on the screen.
My version is not as nice or polished as some i'v seen here but hopefully someone can educate me on the ar film issue as i'm not sure about it and my searches online for it have not been fruitful.
Does the AR film make that much of a difference as my screen is slightly washed out looking. Also, i wasn't quite sure what you all meant by the polariser from the SP, is that the actual piece of glass the led bar is joined onto?
Notice the un-even-ness of the light.
Close-up
De-solder this, DO NOT rip it
The results of breaking the trace and resorting to soldering directly to led. (note that black wire is positive, red is negative. also note resistor in black beside led)
) and mine is fine. As for AR film, I can't find any of the damn stuff anywhere. 
Also, you're using huge wire, that's why the SMT soldering was tough. 
Also, it looks like your LED isn't correctly aimed at the diffuser panel, mine looks fine, not like yours. 
