The key fits into the passenger door and console locks but will not turn.

I know it's the master key because it works in the trunk. The grip is squared as opposed to the oval valet grip.

I didn't consider that the key could be worn down, since it fits 2 of the 4 locks. I'll ask the M-B dealer about it. Thanks for the advice.

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1990 560SEL Anthracite Gray
1990 300SE Arctic White (parts)