Bath fan

[superboiler]

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

[RBM]

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