all you need is a soldering iron (and preferably a desoldering tool as well), a multimeter, and the color led you want (and possibly a resistor).
I used a cheap $10 Ativa brand mouse from Office Depot, and a blue led (3.4V 8000mcd).
This particular LED will NOT be available at your local radio shack, I specially order a batch a while back since it is a low voltage blue LED. (and quite bright too)
You can use any color of LED you want, as long as the rated voltage is 4V or lower and the luminosity is around 4000 - 8000 mcd.
I've recently (Mar-5-08) put a UV LED in a cheap HP mouse I got a while back to mod. After overriding the resistor, the mouse pumps out the 5 volts require to blast the 3.4v led. It needs a small resistor, as it frequently starts to overheat and will stop responding for a few seconds.
Kinda strange, though it makes since with UV light, the mouse works fine as long as it's not on a black surface.
As any kid who's fooled around with a blacklight will know, black objects barely show up if at all. Naturally, the light is not reflected to the eye, and the mouse is blind on very dark surfaces.
Step 1: Disassembly
The screws are usually hidden under the mouse's "feet", or those little black pads.
Once they're out, there may be a locking tab you have to push to pry the panel off.
If you get a cheap mouse, the inner components will be nice and big and easy to work with. the fancy ones will have capacitors and resistors that measure about 2mm x 4mm x 1mm. Those are not fun to place and re-solder.
Step 2: Test Your LED
To do this, plug in the mouse and simply touch the leads from your LED to the same corresponding leads of the one thats already installed.
Make sure you have the leads correct; it is a diode and only works one way.
The red LED found in most standard mice is a 1.7V rated diode, and that resistor is holding the power down to that level so the LED doesn't overheat and burn out.
If your LED does not light up, and you have it positioned correctly, then it is not receiving enough power.
Mine was not getting enough power using this method.
Connect a multimeter to the leads of the installed LED to see how much voltage is actually going across it -both at idle and when the mouse is moved.
If you are not getting enough power for the LED you want to put in, use the multimeter to see how much power you would have if you override the resistor.
To do this, place the black prod from the multimeter on the anode of the led (negative side) and the red prod on the other side of the resistor than the LED is connected to.
Conveniently enough with this mouse, the voltages were (I believe) about 3.2V at idle and 5V when moving.
To test the override, reposition your LED in the same place you just put the multimeter.
As you can see, the two LEDs had consistent luminosities.
If your LED still won't light up, find a lower voltage LED.
If your LED is too bright, consider some smaller resistors to replace the installed one with.
Ultraviolet LEDs have the lowest wavelength, and thusly the highest voltage required to light em.
Blue LEDs are the next down on the list
Followed by Green and the rest of the visible light spectrum in it's order.
Red are just the most common used because they consume the least power other than Infrared. I don't believe infrared light can be sensed by a standard mouse eye though. (some higher end mice do have infrared LEDs)
To find the right resistor without guessing, you'll need some good ol' evil- i mean... math.
You have resistance (r), voltage (V), and current (I, uppercase "i")
The resistance is measured in ohms, the voltage in volts, and the current in amps.
Current = Voltage/resistance -or- I = V / r
the circuit in mice will usually be running at .02 amps. So the resistance you need to drop the voltage by half a volt (.5V) can be expressed by the equation:
r = V / I
r = .5 / .02
r = 25 ohms
so you would need a 25 ohm resistor in that case.
Here's an easy calculator for power and resistance and such:
Step 3: Replacement
It should be fairly easy, especially with big components, to find the LED and the resistor that chokes the power going to it.
Use the desoldering tool to heat up and suck the solder off of the the leads to the LED and resistor (from the underside).
Bend and clip your LED's leads to match the red LED's.
BE SURE TO BEND THE LEADS TO THE CORRECT SIDE!!!
If you need an override for the resistor's spot, the freshly clipped LED leads will work just fine.
Place your pieces in the correct holes and solder them in. The desoldering tool can work for this, but using a normal soldering iron will do a better job.
Step 4: Test and Reassembly
Get the cord back into it's slot through the base, and screw the top back on.
If the mouse's feet will not stick back on over the hidden screws, regular emblem mounting glue, super glue, or even silicone sealant will work to stick them back in their place.
The light is one idle in the 2nd from the last picture, and is active in the last one.