earliest MB with a passenger air bag ive seen was a 1991 s-class diesel (a 350 SD). im pretty sure it was standard, but not available on the e-class in '91 (300D). so to get a passenger air bag youd have to get a later model SD or SDL. no sure how early they had them available.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350