This one seems to be a little different from previous questions on the subject. Here is the dilemma, I don't see a transformer. I have included a few pics.
There are Three wire bundles. For ease we will call them "Bundle-A", "Bundle-B" and "Cannon-Plug" - see schematic diagram for Cannon-Plug.
Bundle-A has two wires in it, Red and White. This bundle comes down the wall from the second story. There is another HVAC upstairs, maybe it runs there?
"Bundle B runs through the wall to the Thermostat. I think it is a 10 wire bundle, but they cut back all of the wires except the 4 wires that are in use at the thermostat.
Here is how it is wired:
Bundle-A-Red is connected Cannon-Plug-Blue.
Bundle-A-White is connected to Bundle-B-Yellow
Bundle-B Red is connected to Cannon-Plug-Red through an overflow-shutoff wire (black in this photo).
Bundle-B Green is connected to Cannon-Plug-Green
Bundle-B White is connected to Cannon-Plug-White
Also notice how the Red/Blue wires are also bridged (with a capacitor? resistor?)
Any thoughts on which wire I tap for "C-Wire"?
On another note, look at the corrosion image. How bad is that? How long before leak and repair - scale 1 to 10?