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If you look at the plug photo Zayed sent you. it's a common frost plug.
The metal is fairly soft, so you can use a chisel to bust through the center of the cap and then force it out with a little work.
Here is a youtube link to show you how.
You are welcome.
Good luck....
I am sure you can get a new plug locally, as long as you know the proper diameter. The MB dealer will know if you show them a photo.