Best bet is an external thermo and two relays. like this:
Derale 16759 Derale Electric Fan Accessories
If not external thermo get the relays, wire up the turnon pin to an ignition only power source, power from the battery and then to the fans.
Did a quick search, check
Post #18:
http://zilvia.net/f/tech-talk/180236...fan-setup.html