The car is 13 years old! You don't say how many miles. I have had this problem on three 300E's. The heat gets to the rubber. They are OK until you try to adjust it, then they fail. So they cost what $150 divided by 13 years equals about 12 bucks a year. At least that's how I look at it.

Buy a replacement and replace the adjustment rod and nut. Also replace the belt tensioner shock. Should be good or another 150K.

Steve