Just to be forewarned people!!
I do this mod all the time, and the other day I got complacent and careless and after installing an LED in a GBA cart it looked perfectly fine, however without a magnifying glass I didn't notice that a tiny little metal dust particle had bridged the positive pin and another pin.
When I put this in my DS phat it bricked it, so I cleaned up the GBA cart, tested it in my GBA micro and bricked that too.
Spending the rest of your day bridging fuses is not fun people!!
It ended OK, I ended up reverting the GBA cart back to it's original self, fixed the GBA micro and got a warranty refund on the DS (cause I needed some cash).
From now on I'm not going to do silly little mods "just because". It has taught me a valuable lesson that it is important not to get complacent with anything you do, no matter how easy it seems.