If it's the same as in my Peugeot 306 (1997) you have to unplug the wires from the therminal-switch witch is located on the radiator.
Then use a piece of wire to short circut the two connectors on the wires. Have the ignition on.
When you short circut the wires the fan will start. (Thats basicly what the terminal switch do when the temprature reaches a certain point)
Best regards
Vegard H Johansen
(NORWAY)