Ok, so I have no voltage coming from the defrost button. It looks like after staring at the schem that the button should provide the 12v to the relay to close the switch. I do have 12v coming to the switch side of the relay, but I am not getting 12v on the other side of the relay to switch it over.
I am leaning to this being a bad relay in the hvac control board. Cause if I have figured it out correctly as I said earlier that power comes from the button that also has a relay.
If that relay isn't switching then nothing else would switch due to a lack of voltage. I am gunna order a new control board and see if I get voltage out to the primary relay. 🤞🏻🤞🏻🤞🏻

 

Thanks for the update! I hope that is the issue! Might also be worth checking you have continuity from the HVAC pin that feeds that part of the relay. Heck, I would be tempted to see if I could force the relay to activate (just to make sure).