98 E300 here. Know exactly what you are saying... mine is under Starmark but it took two visits to get a fix. First visit was a "goodwill" repair of some sort of new lock down bolts under seat - no charge. (DRiver seat only) STill popped and cracked in turns and sometimes stops. Second visit was a more intense repair where rails and some sort of seat underkit were replaced. That DID THE TRICK !!! Had to pay for parts however. Since I had Starmark, they covered labor. If you want the part numbers for what was replaced, email me.

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Barker