Hello Yabaduu,I can just take the hood off. The video I just watched on you tube the guy removed the entire fender. 😂
If your car has factory air con then yuppers the passenger side wing/fender must be removed to access the heater core plenum to mend or replace the heater core. However as mentioned it could be a bad hose in which you might be able to work in a tight cramped space.
If your car is a heater only car, then the heater core plenum is visible inside the engine compartment next to the passenger side apron and changing the heater core is as simple as 4 screws.