For some reason, the Eng GND cable was not connected and tucked away behind the coolant header tank. The coolant level was fine but the header tank was really old and already turned ...
The heater core bleeder pipe was kinked and looked to be slowly leaking from around the bleeder plug. It was super tight so I had to apply heat to loosen it and then found the reason for the kink as well as the leakage. There were so much corrosion inside the bleeder plug as well as inside the fitting ...