Very simple circuit wise, a good diagram in red on here, a feed from the battery with suitable gauge wiring, via a fuse of course as close as possible to battery, to a relay and from the relay to the fans, another feed for the relay (Doesn't have to be heavy duty wire) and use the fan switch to negatively trip the relay, akin to the OE setup

Some go one step further and have a relay for each fan.

The trouble is the switch is one of the main problems as it keeps the fans on for sooo long so really you need to "Uprate" this as well.

Martin