The plastic tab might be off the plastic trim piece. If you take the glass off, thee is a plastic trim piece that goes around the outside edge --- it has tabs that "snap" into the light fixture.

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more