My 1999 Plymouth voyagers cooling fan only works.

With the engine running, disconnect the two-wire coolant temperature sensor near the thermostat housing. That will make the fan turn on as a default in case the engine is overheating and the Engine Computer doesn't know it. If that works, suspect that sending unit.

You can also use a scanner that displays live data to see the temperature that sensor is reporting. The Engine Computer should turn the fan relay on at around 210 degrees, and off at 198 degrees. If it does not, you can use the scanner under the "Actuator Test Mode" to tell it to turn the fan on. Mechanics use that to troubleshoot the circuit when it does not turn on.