I used an aluminium joiner like unfam posted with a 70 deg NO temperature switch in the bottom hose. (http://www.ebay.com.au/itm/THERMAL-SW ... cb83&_uhb=1#ht_500wt_1054)

Min works fine, those switches are good for 10A. Another way would be to fix the switch onto the bottom (outlet) radiator tank.

You put it in the bottom of the radiator when replacing the mechanical fan so it switches on when the radiator outlet water is too hot, i.e. not enough air flow. You put the switch in the top hose when it's an auxiliary fan so that it cuts in when the existing fan isn't doing the job.