Be clear - the car does not need the fan to stop it overheating. The fan only comes on when the temperature increases to about 90C.. normally it will only do that if you're sitting in traffic with the engine running.
If the car is moving, it should not over-heat, fan or not. So wiring the fan in to be permanently on will only help if you get stuck in traffic and the sensor that switches the fan on isn't working.
Ralf S.