Mercedes123 Bumper fell off.

I have a 240D, 1981. The rear bumper cracked off. I found one on the internet, and just bought it... its being shipped.
However, I was told by a former Mercedes mechanic that there was a
problem with the aluminum corroding and breakage, and that there is a lifetime warranty on them.
Is there any truth to this? If so, how do I go about getting a new one?
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