All of the arms are either pressed steel or cast iron. The lower forward arm has a plastic cover for protection, that you peel off and refit onto the new arm.
As far as the number of links.... Mercedes set the standard for ride quality/handling years ago with this 5 link style independent system that everyone looks to. The advanced kinematics were the envy of the world.... What was the USA doing... Solid rear ends and leaf springs!!

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2009 Mercedes ML320 Bluetec