You are marking the firewall. I would guess the heater core if it's back there.
So from my research and ehat you told me the actual heater core is inside the firewall... how do u access it?
Have you checked the carpet under the dash to see if it's wet? I'm not an expert, but the heater core will most likely be in the plenum (plastic/fiberglass) box behind the dashboard. If the carpet isn't wet, check all the heater core lines (hoses) under the hood where they connect to the heater core inlet and outlet. You might try a coolant system pressure test to see if you can find an active leak.
When I had a heater core rot out, I could smell that sweet smell of coolant from within the cabin, and when I checked the carpet, sure enough, it was wet. Passenger-side floor board, under the glove box.
If small block then yes they do. No big blocks don’t have freeze plugs in the back. Kim
