Where is my Common Wire?

Question

I purchased a wifi thermostat and cannot located the common wire. My unit is old and is a Coleman 70 with no terminals inside labeling the wires.

As you can see in the second image, there is a blue wire that is not being used that goes to my thermostat. I am trying to use this as a common wire, I am just unable to determine where this wire connects to.

THank you for the help.

Answer 1

I've highlighted the C wire for you in the schematic.

And in the actual wiring