Radiator Fan 

Electric fans are good but work best when triggered by a thermostatic switch, although some people prefer a manual over ride. You can buy aftermarket fans or hunt through a wrecking yard. Most aftermarket thermostatic switches are placed either through the radiator core fins near the inlet from the engine or are placed in the top radiator tank, and the connections are sandwiched between the radiator inlet hose and the top tank pipe. Most factory switches are located in the engine inlet housing. Adjustable aftermarket switches are available, as are preset non-adjustable.

Ensure the thermostatic switch is placed correctly - one designed for the radiator outlet should not be placed in the radiator inlet as it will be designed to operate at a different temperature. Care must be taken to establish whether the thermostatic switch is normally open or normally closed, and the fan relay configured accordingly.

Fans should be test run first - the first one I used was very noisy which was a feature of the fan blade design. Additionally some fans are designed to run in one direction only - ensure this direction is correct for the side of the radiator/mounting position you will put the fan in.

For maximum efficiency the fan should be shrouded and as large as possible. The fan should be located as close to the top of the radiator as possible and hard against the fins (but not in such a manner that it will move and wear the fins through and cause leaks). The decision to place the fan in front of or behind the radiator and so have it pushing or pulling the air through is based largely on where it will fit best. ideally the fan will e more efficient pulling air through the radiator but you may not have the space to mount it this way. 

Remember the old maxim that it is easy to get air into an engine bay, but harder to get it out - don’t automatically assume you have cooling problems due to not enough air getting to the radiator - it may be a case of air pressure building up in the engine bay and not letting any more air enter.

Multi Speed Fans 

Heaters tend to have multiple speed fans. High speed is achieved by putting full system voltage on the fan, lower speeds use resistors to drop the voltage so that the fan runs slower. It makes sense to use a heater fan resistor. My car has a two speed fan switch, but a four stage resistor. I just experimented until I found the connections which provided the best speeds.