Some cars have fans controlled from the ECU. I joined this forum when my car overheated in a mega jam on the M25. The fan is 2 speed on mine for slow the fan is driven via a resistor and straight off ther 12v for high speed. Usually this only cuts in when the air con is on as well. The relays are in the box in front of the battery and the power resitor is in mid air on the plug at the bottom of the rad. I posted some pics in my gallery.
I bought as complete new rad but just used the resistor module to avoid doing more work than I needed to, and to avoid disturbing anything that was working OK.
You can unplug the relays and short across the contacts, the fan will spin if its OK and for low speed if the resistor is OK