I removed the radio of my 91 300SE the other day. When I reached behind to pull out the radio, I knocked loose a plastic piece shown in the picture. I can't figure out where it was or where it goes. Any ideas?

The piece is about 2" wide along the curve. I didn't notice a part number on it.

Thanks,
Sixto
91 300SE
87 300SDL
83 300SD

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