Trying to wire for Ecobee thermostat

I have an ecobee thermostat I’m trying to hook up, but I can’t figure the wiring out. We have fuel-oil boiler and an AC, with the oil boiler using radiant water pipes (as well as heating our water for the tap) and the AC hooked up to a regular HVAC type of whole house fan. There is no c wire, so I was trying to, on the AC HVAC fan, jump G to Y and then hook the old G wire up to C. But when I looked at the house fan, I saw this:

Am I not looking in the right place, or is there something non-standard going on here? For reference, this is the wiring for the thermostat:

The wiring for the thermostat is G, W, Y, R, and Rc with no C (and the thermostat doesn’t work if you take out the batteries, so I know the C wire isn’t there). What should I be doing, here? Thanks!