I just did the wiring connecting the Cateye buttons to the STI shifters extra plug using the Shimano extension cables. The Cateye buttons come as pairs. You cut off the little junction connector and you've got a pair of cables, one for each button. Inside the cable is an uninsulated wire, and an insulated one (either red or white depending on the button). The Shimano extension cable has 4 wires : blue, red, black and green. Connect the two Cateye uninsulated wires together and to the green shimano wire, connect the red Cateye wire to either the red or black shimano wire, connect the white Cateye wire to either the red or black shimano wire that was not connected to the red Cateye wire. Blue wire is not connected to anything. So, in summary:
Cateye wires Shimano Wires
Since the cateye buttons are separate, once you connect things up, push the buttons and you will see which is up and which is down. The nice thing about the Cateye buttons is that they have the number 1 or 2 on the button, making them easy to identify.
A couple of tips.
I soldered and heatshrinked all connections. The wires are so small that twisting is not a great connection. Each of the wires appears to have a small plastic fibre type wire running intermingled with it. So the trick i used was a cheap battery powered soldering iron. I used that to melt the insulation, melt the plastic fibre, and tin the leads all in one shot. Made doing the connections very easy.
Be very careful removing the outer sheath from these cables. The wires inside are small and easily nicked and damaged. As well, use the tip above to melt off the inner insulation covering individual wires, don't try to mechanically strip and pull the insulation, you'll just break off the wire too.