HVAC adding C wire to older system w/ wire nuts

Question

I have an older system, separate AC and gas heat. Trying to add a C wire. All existing wires are connected outside the air handler with wire nuts. I've unwrapped an unused blue in the lower right of the picture; that runs to the thermostat. As you can see, the other wires in that cable are yellow joined to red from a different cable, green joined to thick green, and red joined to white from a different cable. I believe I can add that blue to one of them - I'm guessing add it to the green/green wire nut? Thanks for any advice