My '79 300SD has wires IN the glass, not a grid ON the glass...........there is a relay that controls power to that.....is it working at all. I have same problem but not have managed to figure it out yet.

Keep in mind the 79 300SD is a W116 chassis

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche