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Remove bottom panel on furnace and tell me if there is a control board mounted to lower housing.If so, there should be a C terminal on that board.
OK we need to run a wire from that C on the board to C on your thermostat.That is 24 volt common to power the thermostat.
If you do not have an extra wire in the bundle between thermostat and furnace, you can substitute the G wire but you will lose fan only capability.
To do this, move the G wire from G to C at both thermostat and furnace ,Then you will also need to install a jumper at furnace between Y and G so that we energize fan on call for COOL.
OK the G wire is currently signaling fan to come on .If you steal the G wire and use it for C, WE have nothing to signal blower to come on. So you will need to take just a short piece of wire at the furnace control board and connect Y and G.
When you removed the door panel. we broke power through door button but you can kill power at switch it you feel safer. Below shows jumper. We just need to send a message to G so blower will come on with AC:
AS long as it is not able to touch anything that might short.I prefer insulated wire like the thermostat wire. If the thermostat wires are long enough in furnace, I would steal a piece a couple of inches long from one of the existing wires.