It appears on my 124 you should find wire contacts for the eliments behind the right and left panels between rear window and upper rear door windows.

To save from removing those panels, just a guess, but I would remove the rear seat bottom and look to find a wire harness on both sides leading up to and behind these panels. If there is, I would tap these wires to see if you have voltage while switching rear defrost on and off! It may tell you something, and then again maybe not! My bet is that someting elsewhere is amiss!