This shows you how to add a right mouse button to a Macbook's trackpad -- click the left side of the existing button, that's left click, click the right and that's right click. Works in Windows, Linux and Leopard.
Step 1: Find the Electronics
While you don't have to take the top panel out (especially on a Macbook Pro) since you can reach through the battery hole, I find it's easier to do so. If you don't, make sure to disconnect power!
Make sure you have plenty of light to work. This modification requires only a few soldering joints (as few as two) but one of them can be pretty difficult to do, so be sure you can see what you're doing.
Remove the transparent plastic that protects the circuitry; it can go back on later.
Step 2: Find the Test Point
The large chip there is the microcontroller that interprets signals from your keyboard and touchpad and converts them into mouse position data; it also reads the button. Note that the keyboard has to "talk" to this to work -- unlike desktop Macs in which the mouse connects to the keyboard, here it's the opposite.
Now, look at all those gold-colored dots on the circuit board. Those are test points that can be used by technicians to access various functions of the hardware to test it. Note the small wire going to the touchpad button -- that also leads to a test point and eventually to the microcontroller. The button shorts to ground when pressed.
If you have a voltmeter you can verify this by setting it to continuity check and touching the trackpad's chassis and any of the 4 small contacts on the button; two of the wires will "beep" always, the other two only when the button is pushed.
Near the main chip, look for 4 test points placed like the arrow keys on a keyboard. You want the "right arrow" in my picture. Pulling that to ground marks a secondary click (aka right click) event to the microcontroller, which dutifully reports it to the rest of the laptop.
Step 3: Find the Test Point for the Primary Button (optional)
You can probably look around for more functions.... A possible variant of this mod is, instead of leaving the main button alone, move it to the opposite corner of the button. This means you have to basically do what I describe twice. In that case, you want to disconnect the existing button and use another test point.
Step 4: Get the Right Sort of Wire
Now the interesting part. You want to get a very thin wire, as thin as possible -- if you know how to remove insulation enamel from a coil wire, use that. If not, use one of the wires from a phone cable, those are plenty thin but still stranded rather than solid so they can take a bit of repetitive movement.
With a soldering iron, let the wire end "soak" some solder in, then cut the tinned (soaked) part until only a bit is visible from the insulation; this should help prevent shorts. About six inches of wire is more than enough.
Step 5: Solder the Wire on the PCB
Okay, now here's the hard part. You need to solder that end of your wire to the test point you found earlier. The best way of doing so with a normal iron is this:
put a little bit of solder on the iron.
"poke" the test point so that some solder remains on it.
hold your wire end against the test point.
poke them again with the iron so that the solder melts together. Keep it there for about half a second.
This is critical and it's also the only time where you can break something, so be sure to have plenty of light and be calm :) I recommend holding the soldering iron vertical. Use the thinnest tip you got.
Step 6: Connect, or Make, a Pushbutton
I didn't have any microswitches that would fit the thin space between the edge of the button and the battery, so I decided to make my own. Fortunately, we're just pulling a test point to ground, and guess what's grounded? The metal parts of the trackpad assembly. This along with the way the button is built makes putting a switch together super easy.
Step 7: Fix Your Switch in Place
Now let's make the switch more mechanically solid and protect it from shorts. If you used a microswitch in the previous step, you should have a good idea of what I mean. Tape and glue are your friends here... it's amazing how much of even the macbook pro is held together with double sided tape as it is. Keep in mind that in normal use this will be pushed against the top of the battery, so it doesn't need to be THAT strong. Still...
When you push the corner of the button, the (metallic and grounded) back of the button will touch the wire you added.
You're done! All you need to do is close it up.
Step 8: Close It All Up!
Put the piece of transparent plastic that protects the electronics back in place, use another layer of tape to hold it there if you have to.
Note that on my particular laptop I didn't even need to open it up... (Well, on MY particular laptop I had to because while I was at it I also cleaned the fans).
Video of it working (in case embed acts up) : http://www.youtube.com/watch?v=PPjDi4E_c3A
If you like my work, check out my autonomous robotics page at http://www.robotseverywhere.net/