Have an existing Nest doorbell on one door, am trying to add one that belonged to a family member. Tried to connect and chime worked but would not connect to my Nest app. Tried restoring factory settings but flashing blue never turned yellow during reset process. Then I learned the chime box setup was skipped since we already have one installed, and my friend didn’t think it was necessary. Attached is a photo of our current chime situation—I removed two black wires from the rear connection(the one I’m trying to add) prior to this photo. Which black wire do I connect and to which of the white boxes? The one with the red wire? Thanks for any suggestions! 
