Well Well!
Ok, I looked at the fuses... none blown....
But I did notice that when you hit the defrost button, the fan did not come on fast as it did before....
So, I looked at the fuses again... wiggled and rotated #7... and hey presto... defrost mode working again!
Wow, good call MBshop... Thanks!
Now, I will see if the thing still blows hot tomorrow!

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2009 Mercedes ML320 Bluetec