Front is cooling okay...?
I'll assume so.
Rear engine coolant flow control solenoid valve has failed/open.
Refrigerant flow through the evaporator expansion valve is CONSTANT (hissing) due to the nearby HOT heater core not allowing the evaporator vanes/fins to decline to the 34F temperatrue wherein the expansion valve will begin to block refrigerant flow.
If i am correct you should have good rear cooling capability before the engine coolant rises to operational temperature.