Richard28, keep in mind have a wagon. I had the trim removed on the rear hatch and knew that all the wiring had to go through the rubber "elbows" at the hinge area. I removed the interior trim piece in the ceiling and found a junction point where about 14 wires were attached. This junction point was where the wire runs from the front of the car met up with the wiring going to the hatch. Using the wiring diagram from the service CD I found out what wire number was for the rear defogger. Using a meter I found out that I had voltage at the junction point but not from there to the defogger element. I also ohmed it out and I got an open between the junction point anf the defogger element, which of course meant a break. I removed the screw holding down thw wire and pulleed on it and there it was, a clean break.
I see that you have a sedan, so I would suspect that you may have a similar junction point but it would be mounted on the underside of "hat shelf" in your trunk.
The wiring diagrams don't really tell you where exactly the components are.
Ok, I just checked the wiring diagram for sedans/coupes, there is no junction point for these models. It is a straight shot from the relay to the rear heating element. The wire is black and is a 12 gauge wire.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member