In an earlier post I wrote about my attempt to take out a Synaptics Touchpad form a laptop and connect it to my computer's PS/2 plug.
The Synaptics Touchpad pinout seems to be different for each model, so be careful not to fry the circuit! Triple check every pin and connection before you power this up. Also remember that the touchpad is not plug and play: you need to connect it to your computer before powering it on.
I have used a Synaptics T1006 0544 touchpad.
The pinouts for the PS/2 keyboard connector and the Synaptics T1006 0544 touchpad can be viewed in the schematic below.
You can observe in the schematic that I only connected the communication pins (Vcc, Ground, Clock, Data).
The pins Left Click, Right Click, Scroll Left, Scroll Right, Scroll Up, Scroll Down can be linked to external buttons (I haven't done that because the Synaptics software allows default scrolling and normal tapping).
For the right mouse button you can define a corner of the touchpad that when tapped will trigger the right mouse click action.