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