Possibly damaged upon installation or maybe part was on the shelf for a while before it got to you. What I do for all the rubber bushings and boots on my 201 is use very thick (30,000 cst) pure silicone oil to coat them. That way they don't deteriorate as fast. I actually use it when I shine my sidewalls after a car wash as well. Also over my spare tire to keep the rubber supple. Silicone doesn't degrade rubber like petroleum based products so it's safe.

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1990 190E 3.0L