There has been a post about this in the diesel section started by silverbullet...
The rear defroster of the cheaper mercedes models can easily be tested; the W126 has a different type of rear defroster. Mercedes uses a defroster with very tiny sinus-waved elements in the more expensive models and these elements are stuck between the different layers of the rear screen.
If the top layer only gets defrosted, there is something wrong with the rear defroster itself. The lower 2/3 part of the elements is disconnected from the power (left-side) or ground (right side) connector.
This will be quiet difficult to repair; you can have a look at the connector side to see if there is anything wrong, but I think it will not help you...
Maybe someone else on this forum can give you a more positive answer...
good luck & greetingz,
with a minor 600+k