My path might sound crazy, but it works well.
Used an early BMW Z3 4 cyl fan switch (two speed). Cut the brass fitting out of a scrap yard BMW and soldered that to a brass "T" fitting with a pair of hose barbs.
Routed the transmission fluid from the in-radiator cooler through the "T". on it's way to the external trans cooler. Since the "cooled" trans fluid is essentially the same temp as the radiator coolant, the sensor offers pretty good control of the fans.
Added a dual function A/C pressure switch from a Windstar. With the A/C on, the fans are on "low" if the A/C pressures rise above the Windstar's 1st set point, the fans kick on "high" and ultimately, the Windstar's pressure switch could shut down the a/c at it's upper set point.
If you don't use the transmission oil cooler, it'd be pretty easy to mod the trans cooler line fittings to add a temp sensor to one, block the other off and then open the trans cooler inside the radiator to the engine coolant to get engine coolant temps to the sensor.
Also, check out "Klixon" temp switches -- available in a variety of styles and temp settings -- would be easy to clamp (or glue) onto the radiator or trans cooler line fittings for a basic "on/off" control of the fan(s).
I would suggest a very heavy duty relay to control power to the fan(s)!!! Think I spec'd out 60 or 70 amp units from Waytek Wire if memory serves.
Not super in-depth, but here's a short post on my system:
http://www.therangerstation.com/forums/showthread.php?t=161376