Permatex makes a repair kit for rear window defoggers. There's an article about using it here- http://www.rd.com/familyhandyman/content/55830/

I don't think that kit will work. That is for the type of grid that is painted onto the surface of the glass, with grid lines that are about 1/32" wide.

I don't know about the other W123 models, but on my 300CD the lines are REALLY thin and there are a LOT of them - about 1/4" apart running top to bottom instead of side to side. I haven't looked closely enough to be sure, but it looks as if they might actually be hair-thin wires imbedded in the glass.

If that's the case, the permatex kit wouldn't work. If I'm wrong, I'm sure others will chime in and correct me...

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

Thanks
I have one of those kits that is still good.
I also found I had 2 others but the liquid had evaporated from them.
I have used that kit on my gassers that had the scratched grid problem.

I found out when I bought the last one it is better to bring the part number and the maker Permatex along with you as Autozone an Kragen (Parts America) Counter Guys were not able to tell me if they had it or not from my description.
However, an independant auto part store had it.

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84 300D, 82 Volvo 244Gl Diesel