I believe your cab rear window is a lot like the one in my 1680, and it was a pain.  I wrapped a towel around a heavy yeardstick, and cleaned it that way, occasionally changing towels when they got dirty.  Tried several other things with long handles and spongy things on them, but the towel and yardstick were about the best compromise to keep it clean.  When the sun gets just right in the evening, it is very hard to see what is in the grain tank on a dusty day, isn't it?

A sponge floor mop worked about as good for us on front and side windows on the cab, got that trick from a farm magazine several years ago.  There is a time close to dusk when the sun reflects on soybean dust on the side windows and makes it impossible to see the header, a light touch with a towel to knock the dust off helps until darkness comes, and you can see well again.  Some say the california car dusters work well for dusty cab windows, and they could be used on that back window as well, but I never tried them.