Phil, I just repaired the rear defroster on my '87 TD (W124) and learned a few things. One the light in the defroster switch is a LED and is independant of the rear defroster. By that I mean, my rear window defroster was not working yet the light was on. My problem was a clean wire break towards the hatch. So the light in the switch is not indicating that the rear window defroster is working, just that you hit the switch. So with all that said, for your light not coming on, it could simply be the switch. Is this how you figured the rear window defroster wasn't working, or was it not clearing up?
At the moment I forget which fuse runs the rear defroster.

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