How to add "C" wire using a/c fan wire?
The Honeywell video describes the process, when the thermostat is used to control a forced air furnace. Since your situation is different, this may not work for you.

If you want to try it, you should proceed as follows...


Disconnect the green wire in the second photo from the G terminal, and connect it along with the white wire to the C terminal.
At the thermostat, move the green wire from the G terminal to the C terminal.


NOTE: This may not work in your situation, and may void the warranty of the equipment.  Proceed at your own risk.

In this configuration, the thermostat will not be able to control the fan. And depending on the thermostat, the C wire may have to come from the same transformer as R. Using C from the Rc transformer may not work.



I've labeled the wires, to hopefully help you understand what's what.



This is what you have now:



This is what it will look like after you switch the green wire.