There are multiple ways of doing it but basically you need two things, (a) a thermo switch to turn them on when the radiator gets to a certain temp, you can get adjustable switches or you can look here:
http://www.outerlimits4x4.com/ftopic147839.php
You can get adaptors that go in the rad hose close to the thermostat, with a threaded hole to put in a sensor.
Secondly whenever the a/c is running you need at least partial fan action ie at least one fan running at at least half speed. This may be possible to trigger from a pressure switch in the a/c circuit (there may already be one) or just hook to the compressor.
Having said all that, I'm not in the know with intercoolers and whether it's worth running the fans with some other trigger besides your radiator coolant temp.
This is not legal advice.