I'm not familiar enough with the W115 era to know if they have a bowden cable on them or not, but you do have a linkage that comes into the cab for you to select the gear. Make sure it's actually going into "D" and not some other gear.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)