Can I use an unused wire as a "C" wire?
The "C" wire is typically a common, or the other side of the 24 VAC transformer. The wireless Tstat would need the power source to operate. If there are unused wires in the wiring harness at the thermostat, then using one of these wires for the common is acceptable if that wire is clearly marked (as common). This wire would connect to the other side of the transformer that supplies the 24 VAC (follow the red wire back to the transformer and see how the other side/wire is connected. does it go to ground? does it leave the transformer to somewhere else?).