I had the same thing on my '86 560SEL... I determined the squeeks were from the plastic threshold rubbing against the vinyl door material...

SO i glued a strip of satin ribbon along the bottom of the door, same color as my interior.. dark blue in my case.

problem solved.. that was almost 10 years ago and the ribbon is still there. i used spray on upholstery cement

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K