I've recently developed what i believe is a problem with the rear windscreen in that the top portion doesn't de-mist / de-ice.

I'm sure it used to, and i've only noticed it in the last few months (ive had the car for nearly 2 years now). Mercedes INSIST that the top 5 bars are actually the arial, although i believe the arial runs vertically on the back and you can see where that is. it IS coincidentally the same region of the screen where the arial comes down to, so could be related, but i don't believe them that it is designed to be this way and shouldnt clear as they arent' heating bars, but arial bars!

can anyone confirm please whether they have the same problem in this frosty weather? does the whole rear screen clear evenly, or does the top portion struggle, or not clear at all?

 

I noticed that mine does the same thing. The car is new to me so I assume that the dealer answer is correct.