Siemens KG33NX74GB Fridge/Freezer

I have a Siemens KG33NX74GB fridge/freezer which is not draining the defrost water from the evaporator drip tray and the excess water is running down the back of the freezer and icing up on the bottom right of the freezer. The pipe which is joined to the tray mounted onto the compressor is clear. I therefore need to check the outlet from the drip tray itself and whether the defrost heater is working. Does anyone know how to get the plastic housing apart which surrounds the evaporator coil? Any advice gratefully accepted.