C wire problem for WiFi thermostat
A wiring diagram would be helpful. In leu of that , the transformer has 4 terminals. Two on the high voltage side and two on the low voltage side. It looks like the black and yellow wires on the transformer are the high voltage wires. The other side has a red with an inline fuse and a brown and green in the same terminal. The green goes to ground. The brown should be the common. There is likely a convenient place somewhere along this wire to hook to. The pictures aren’t too clear however so this should be verified with a wiring diagram.