How can you connect the radiator fan to turn on went you turn on the car?

This is not necessarily recommended as you will not have any control over the fans operation -

But to answer your question -

Run a new wire/cable from the Ignition circuit (through an inline fuse) to the power cable to the radiator fan.

Select the 'Earth Lead' at the Radiator Fan and connect this to the vehicle Chassis (or bodywork).

The Radiator fan will now run constantly all the time the Ignition is switched on!!

N.B

You must choose a cable that is suitable to supply the power to the Fan - so you will need to do some basic 'Ohm's Law' calculations as this will depend on the 'rating' of the Fan (check the small label or markings on the Fan for it's current consumption) - this will also determine the size of the fuse that will be required....

This would be best left to someone who is competent and capable when dealing with vehicle electrics - there is a possibility that it could end in disaster!

It would be best to install a relay in the wiring so as not to overheat the circuit