It sounds like your self dimming glass has failed. The one for my '97 was $230 aftermarket, but yours might be a bit higher for your model. I have read that you can cut away the plastic rim around the outer glass, and remove the outer glass and the goo inside, leaving behind a silver heated mirror, but it might be easily scratched if it is silvered on the outside of the glass. I seem to remember that the glass from MB was $575, but that sounds so absurd that I must be wrong.

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'97 E 300 D