As a relatively new home-owner, I recently came across the whole C-wire issue for the first time, having had to recently replace an old thermostat, and there is no C-wire connected inside the furnace.
The furnace we have is the Goodman GMP100-4 and is 25 years old, but the last homeowners replaced the circuit board in 2015. There are two unused (black and blue) wires in the 18/6 wire that goes from the thermostat to the furnace, and I wish to use one as a C-wire.
Please could you help me out with where this would go in the furnace? The circuit board is visible in the attached pictures along with the whole set up. I'm fairly handy in general but new to this particular situation/electronics.
I really appreciate any time you can give. Thanks!