My understanding is they come on when either the engine gets up to about 105 at fast speed, or when the aircon is turned on, but spin at a slower speed.

You can trick the temp sensor by unplugging and shorting the cable, which makes it work as a closed switch. Mine didn't work originally but a few smart taps on the spindle of the fan and a dose of WD40 did the trick. I drilled a tiny hole in the fan cover and sprayed through that, and sealed up afterwards.