I am replacing my existing thermostat with a new smart thermostat that requires a C wire connection and the old unit did not. I intended to utilize the unused Blue wire which is not connected on either end. The thermostat connection is straight forward but inside my furnace there is no control panel and no wiring terminal to connect the blue wire to. From what I’ve read so far I may be able to splice into the 2 twisted white wires coming from the condenser unit and some other device but I’m not sure if that is correct.
You will need to connect the blue wire for "C" to the wiring node that corresponds to the "BL" connections at the transformer as indicated by the yellow highlight in the following picture.
Do not connect to those two white wires unless you first confirm that the also connect into the "BL" node at the transformer.
Note from the picture that this "BL" node is also connected to the ground connection of the furnace. This is most likely the metal frame of the furnace so you could look also for a screw that attaches to the metal frame and attach your blue wire there.