Here’s a great wiki site showing you good information on how to hack the ASUS Eee PC including how to add a GPS unit without breaking your arm off.
There are several points on the motherboard where power can be draw, for devices requiring 3.3v or 5v. Ground is universal and allows for short wire runs. A diagram over the points can be found on the research page.
The signal wires thickness is 30 AWG and 24 AWG for the power and ground wires.