Blown head gaskets lead to a weakened heater core
But it makes sense if you think about the systems and how they work.
The head gaskets seal the head to the block. The head gaskets also separate the combustion chamber from the water jackets in the head. The water jackets are passageways where the coolant travels through the head surrounding each cylinder. When the head gasket fails between the combustion chamber and the water jacket it causes combustion gasses (and pressure) to escape into the lower pressure cooling system. When the cooling system is over pressurized it causes the heater core to weaken and eventually leak. This will require the heater box to be removed for access to the heater core and AC evaporator.
The heater core in this unit also shows evidence of calcification in the coolant tubes. The coolant should be a clear color coating the tubes, here you see a whitish tint. While you are changing the heater core be sure to inspect the AC evaporator. If it is face plugged that will lead to decreased airflow from cab AC vents. It is a good time to replace it if needed while you have the heater box out.