Bath fan

I want to replace a bath fan with one that has an automatic humidstat. To do this, the fan requires three wires - white, black, and red. This will allow the fan to be turned on manual or automatically. I only have a white and black with the current fan. I need the red wire. Can I:
Just run a third single red wire
or
Do I need to feed a separate three wire set to the fan (take out old two wire)
Thanks
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You must run all the circuit conductors together, so it's probably easiest to remove the two wire cable and run a three wire up from the switch

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