heater on zafira not demisting front screen?

My heater used to work fine, zafira 2001 diesel, now after been parked for 3 months in a tilted position, the front screen mists heavily and although the heater works full speed it can't demist it, especially in rain. the drivers door had some water coming in from the bottom. its likely a rainwater drain is blocked but where do i look. i don't have a sunroof. please help as rain driving at night is dangerous.