If the key is the correct key try some lubricant ie silicon spray to free up the lock tumblers they can get stuck. Graffite is the correct lubricant for most locks but check with your MB dealer. Don't use WD40 as it is also a corrosive. (Dear ol Dad, was a locksmith for Sac State University for over 20 years)

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)