I am adding a WiFi thermostat. I have R,W,G,Y wires. Plus 2 extra coming from upstairs that go between humidifier and humidistat. So I ran an extra wire for the C wire. When I attach it to the C terminal on circuit board, it blows the 5 amp onboard fuse. The white wire that is already attached to the C terminal and the red wire on the G terminal go to the outside A/C unit. Could those cause a short when adding the new wire? I have a tester but not sure what to check. It could be a bad thermostat. How do I test for a short on the thermostat terminals?
Here are some images without the new wire connected: