mists

Hi, I have a really annoying thing happen on my CLK (2009 plate convertible) which is when I switch the heater on, the front windscreen steams up and where I could previously see out of the window, I now see nothing for about a minute whilst it demists. I also notice that sometimes when I'm...

If I turn the air-con off, or re-start the car (several hours) after a run with the air-con on, the screen mists up. I understand that the evaporator is likely to be covered in condensation, but is this behaviour normal. I'm wondering if the evaporator drains are blocked so the condensation...