How do I connect the spare C wire to the old Lennox system (model # lennox g12q4-110)?
I have this exact same furnace (30+ years old) and very similar messy nest of wiring. As user76730 noted the yellow on the 24V side of the transformer is the key. In your photos, this is the left most wire nut that is pointing up with 2 yellow wires and a green attached. I had a spare blue thermostat wire from my old Honeywell mercury blob setup that was completely unused and just wrapped back in the wall. Hooking that up to the yellow bundle and then the C terminal on the thermostat is working like a charm. I have a Nest E with just the R and W, and it was power stealing badly. Now it runs very smoothly.