About a year-and-a-half ago I did a automatic, thermostatically-controlled fan mod that I developed from the simplistic manual switch fan override developed and presented by others on this and other forums.
The real trick is the diode added into the relay as it prevents the check engine light from coming on as occurs without it.
You don't have to do the whole automatic thing with the auxiliary relay, the thermostatic switch, etc., etc., just the diode added to the relay and a ground switch on the "ADD WIRE" lead extended to whereever you place the switch. You can flick the switch anytime you want the fans turned on or off. I automated mine to eliminate my having to do the switching manually.
The relay, diode, switch, and some hook-up wire will cost you less than $20. The fully automatic setup costs $65.
Recently I got a Predator ($350 +/-) for its performance-enhancement features and other capabilities, which eliminated the need for my automatic fan controller. I left it installed as a backup in the event I should need to re-install the stock tune.