At least on my '83 300SD, there is a strap that runs the width of the speedometer section that hits pads on both the tach side and the small gauge side. There was evidence of corrosion on the contact points. My preferred cleaning method is to use a pink pencil eraser or a fiberglass scratch pen. The fiberglass scratch pen is meant to remove rust and debris from scratches prior to painting. It's not super abrasive but can go through very thin plating while the pink eraser won't but it also won't remove heavy corrosion. After cleaning, it put a tiny drop of Deoxit on the contacts. You only need enough to cover the contact point, don't flood the area. The spray can is a bit harder to control than the fingernail polish sized bottle but the can will reach into switches better.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120