In my case, I used a 3 position switch, and wired the down position as auto to the DC Controller that Bounty mentiones (it has a cable for the IGN, which switch the fan off if there is no current), and connected the DC controller to the high speed cable of the fan. In the up position, the switch powers a 40A relay connected to the low-speed wire of the fan, so if the DC controller ever fails (it's worked flawlessly for 2 years so far), I have a back-up.
For the controller, you connect botht he negative and positive cables to the battery, the blue wire to the AC compressor (so that if the compressor turns on, the fan does too), the yellow wire to a 12V source (you can go to the battery if you want the fan to continue to turn even when the Jeep is off, or you can go to an ignition wire, or to a switch connected to an ignition wire, as I did, so I can turn the fan off for river crossings and such). Then connect the negative and high speed positive fan wires to the controller. For the relay, I also connected to the battery as a source, and connected the output to the low speed wire as stated before.
As for the temp, I set the DC Controller to maintain 180* temperature (keep in mind that the controller reads the temp AFTER it has gone through the radiator), and thus it keeps the Jeep at about 195* regardless of how hot the day is (and I live in Miami, so we get plenty of those here). The controller comes with this setting as a default, so you do not need to ad any jumpers to the controller.
So now, I can have the fan turn automatically (DOWN postition on the switch), turn it off (MIDDLE position of the switch) for water crossings, or override the controller and run the fan at low speed (UP position of the switch) which is more than enough power to keep the Jeep cool even in 90*+ days.
Felipe
