With the plug in place, and the FWD fuse installed, turn the ignition to "on" but don't start the car. Jam one of your multimeter probes in the back side of the connector (in the appropriate terminal of course) and the other to a good ground; usually some bare metal of the car chassis. My meter has pretty long leads, so I ran the negative probe out to the factory ground points in the engine compartment.
When done properly, you should get 8.5 volts or more. If your meter is like mine, the probes are too fat to get into the connector, so you may have to jam something metal in there to get a decent connection, and then touch your meter probe to that to get the reading - I chose a small pick.
Now, remove the FWD fuse, put the gear selector in 1st, fully open the throttle, and repeat the test. This time, you should get less than .5 volts. If this checks out, you've got the right wire. If not - use the wiring diagrams and diagnostic sections of your FSM (you did dowload one right?) to troubleshoot and track down the right wire.
If you get totally stuck, post here and i'll try to help as best I can.
Now it's time to cut the wire. Dig a few inches of wire out of the harness and snip it.
Now we can extend the wire out to the switch. I located mine back near the handbrake, but you can put yours wherever you like. Just splice two lengths of wire to extend each side of the white/green wire to your switch location. Solder and heatshrink the connections.