Help connecting unused C wire for Smart Thermostat
Your C connection is as depicted below in the furnace electrical diagram as I highlighted by the blue line. As you will note that highlighted connection in the lower right corner is to connection #5 of the transformer. Note also that that connection also connects to the frame ground of the furnace.



It is very typical that your air conditioner compressor is driven  from the furnace control wiring with a two wire cable. One of those two wires will also be connected into this C wire connection. If you carefully trace your wire connections from the AC unit you will find the wire of the two that ends up being tied to furnace frame ground. (With power turned off to the furnace a digital multi-meter in continuity testing mode can be a handy way to find the wire that connects to the frame ground).

Once you identify the wiring harness that attaches directly to the frame ground (or transformer connection #5) you can tie your spare wire in the thermostat cable into that harness wire.