It would be much easier to just use the 3 wires that are already there rather than running wires through the steering wheel column. god_of_cpu has an app that takes the joystick input and maps keyboard shortcuts (in the link above), and I have a winamp plugin for this setup as well and a girder plugin (that doesn't work right).
Oh, and there are USB touchpads that work just like a mouse.
cheap better . There are more but that was a quick search.
EDIT: Oops, those are both serial!