I have had this problem 2 years in a row. The easiest way is to take the trim panel out, dremel out from inside. the new inserts are about a dollar a piece, but you need a special tool to get that installed on to the tailgate.

As a preventative measure, either you switch to plastic/nylon screws or lube the hell out of the metal screws with anti-seize paste before screwing them back to the holes.

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98 ML320 (gone but not forgotten)