The key here is the word aftermarket rails. I have seen what MB DOC has said. The aftermarket rails will fail. I have had the Aluminum ones have the rubber that is bonded to them come off. When this happens, no one knows. They drive the car until it makes a noise, TOO LATE.
I will agree that the FACTORY alum rails were nice and rarely failed....But we can't get them anymore.
I have seen Plastic aftermarket rails fail with 20k miles on them. Just be sure to use the Mercedes Original rails, and you will have the best product I have seen.
I hope we will all agree on this.
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