After countless hours of not finding the # 3 circuit short (but I at least I am now more familiar with my car) I decided to unwrap and check all my new radio connections - and there it was - I had mistakenly joined what I thought was a ground which looked like a brown wire but was in fact a brown striped wire. I cut this wire and NO MORE SHORT.
I now have a mystery wire which is BROWN with a BLUE stripe - this wire went to the centre 5-pin plug at the rear of the old Becker radio. What should I do with it???

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1986 300E

Probably was for illumination along with the instrument cluster. If your new radio has an input for nighttime dimming, hook it there. Otherwise just tape it up.

And congratulations on finding the problem.

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1989 300 SEL that mostly works, but needs TLC