I was bored tonight with nothing to do, so I went surfing through the mod thread, and came across the mouse led mod. I decided to try this on my mouse, but as you can see the entire bottom is all red plastic already.
Well I noticed in that thread someone added another led to light up the roller ball that would only come on when the mouse was moved. I figured I would add this blue led in the mouse for that purpose.

I figured that if I just soldered it to the exposed led leads that it would work just fine. The battery you see in the pic running the led is a 3v lithium battery. With the batteries in but the mouse in pieces I tried touching the wires to the LED leads I swapped the wires, and either way I tried It didn't come on, and I'm not sure why. Heres a pic of the led placement in my mouse.
I'm hoping that the led will come on when I scroll the wheel, but if all I can get is for it to come on when moving the mouse that is fine too.