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It isn't the temperature alone. It's the Watts or Joules that are not high enough for large pieces. Your typical electronically controlled iron can be set for 850f and that's more than sufficient. But, for a large volume you might need 500W or more and most irons that can reach the required temperature are in the 100 to 150 Watt range. Sufficient for wire to wire or small tab to wire.
Although fan speed is controlled by a resistor in series with the fan motor, those fans draw a serious amount of power. You would need to find the ratings of the fan speed switch in your car to find out what values they used. A 10W to 25W potentiometer can be purchased after finding out what values produce minimum speed for your fan. You could get several 10W 100 Ohm and 10 Ohm resistors and experiment. No resistance will be maximum speed, obviously. To use a much lower power resistor you will need to use a transistor with a heatsink or an encapsulated fan speed controller module. Like this one available on Amazon: ELEGIANTFurmores3317 This is a 90W PWM motor speed controller that will run from 12V and cost about $7 US.