How to clean the blocked water tank of Chery qq3 heater?

The heating box of this car has two pipes installed on the dashboard of the firewall. If it is cleaned, it needs to be removed, with one end connected to tap water and the other left outside. If it is replaced, it will be more troublesome and the workbench needs to be lifted. This situation has nothing to do with the slow rise of water temperature, generally because the thermostat is too open or not installed. Need to check the installation