further question on this. i've been thinking about this idea too, but i was wondering how you could hard wire it to a switch to turn it on whenever you wanted, but also leave it connected to the thermostat switch. that way you wouldn't have to worry about always turning the fan on and off when your just cruising, but you can turn it on when you want extra cooling.
if you want to find the positive, just find the connector to the fan motor and unplug it. turn the ignition to run(but don't turn the car on) and turn the a/c on(just hit the button, this turns on the fans). using a voltimeter, stick the positive lead into one of the side of a connector and put the negative lead on a ground. if there is a current, you found the positive wire, and vise versa.