I'm trying to figure out where to connect a C wire from a thermostat to my furnace. I found this old post for the same furnace type (Trane XE70), but it seems as though a red wire goes to the terminal the other poster had a white wire go to. This made me too nervous to try it without inquiring with more knowledgeable people.
The wire bundle with the blue wire is the one coming from the thermostat and the wire bundle with the wrapped unused green is coming from the AC unit (I believe). Any suggestions on which terminal would be a common ground? Is there an easy way I can test with a multimeter?
EDIT: Requested wiring diagram for the model.
Thanks! - Sincerely, DIY Noob