Getting a replacement key could be a real headache, depending on your locale and the proximity to a Benz dealer. Most dealers will not make a replacement key unless you can prove ownership of the vehicle. I assume, since you have not been able to start said vehicle, you probably haven't had it registered in your name.
If that is the case, you will have to either get the registration in your name, or provide ironclad proof that you are the individual entitled to the "inheritance" of the vehicle. Aunt Polly's deathbed wish probably would fall short, though, an actual written will, bequeathing the vehicle to you, could possibly get you by.

In any case, you'll need the proof of ownership and proper picture ID. Good luck, and let us know if you have any positive results.

It might be less a headache to search eBay for a replacement ignition switch, with keys, and if you are in no hurry, or you would more likely find what you need at Fastlane. These are good folks, and help support this forum. You can find a link to Fastlane on the homepage, here, along with a toll-free number to call for assistance.

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1980 300SD 146K miles.