The resistor on the cooling fan allows it to operate at 2 speeds. In the stock configuration, the cooling fan uses a low and a high speed relay.
The low speed relay is activated by the thermo sensor located at the bottom of the radiator just below the cooling fan. When the thermo sensor sees 185 degrees, it completes a path to ground for the low speed fan relay and activates it. The low speed relay then supplies the fan motor with a path to ground through the resistor, causing the motor to turn at low speed.
The high speed relay is activated by the engine coolant temperature switch which is located on the water neck. When the switch sees 215 degrees, it completes a path directly to ground, bypassing the resistor. This allows the fan motor to turn at full speed.
The easiest way to test the fan circuit is to jump the terminals that plug into the thermo sensor located at the bottom of the radiator. With the ignition key in the ON position, unplug the connector and jump the terminals together with a wire or something. This will simulate the thermo (fan) switch operating. The fan should kick on. If it doesn't, you probably have a wiring issue.
See these threads:
click
click
Here's the diagram: