I was wondering the same thing when I got my 1985 300D. I was the 2nd owner and had theowners manual. When I looked in the owners manual it told me how using the key in the passenger door would NOT open the other doors.

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Frank
84 500SEL EURO 101K ( JUST LIKE MY 1ST WIFE. GLAD TO GET RID OF HER! )
85 300D 310K (sold)
90 350SDL 184K sold
83 300D 118K (sold)
88 300E 153k (sold)
93 400E 105K (sold)