Yeah I absolutely love my back-lit GBA! So comfortable to play on.
I just finished modded one recently, still working on finishing up my second one (have to get a new ribbon adapter since the guy I bought it off sent me an incompatible one).
I decided to spend a bit more money on getting the ribbon cable that has the 5-brightness settings, via toggling Select+L. I didn't want to put any janky switches into the case, and I prefer having more dimming options than just the 2-settings that a majority of ribbon adapters out there have.
The mod is pretty easy. Really the toughest part about it is cutting the case tabs inside so the GBA SP LCD fits:
Here are the tools I used for that:
Three wires to solder:
And voila!
*Bonus photo: This is what my second GBA it looks like with the incompatible ribbon adapter. I have the proper one on-route to me now: