The OE plastic license bulb holders in my 1984 300D fell apart. I stripped a 1983 300D last spring but couldn't find its holders, so they likely crumbled too (forget). Anyway, I found an LED assembly which fits, so file this info for the future. I browsed ebay and Amazon until I found one which fits the opening and screw holes. Cut your own gasket.

"VIPMOTOZ Full LED License Plate Light Lamp Assembly Replacement For Toyota 4Runner Sequoia, 6000K Diamond White, 2-Pieces"
$17 for qty 2, free shipping (needed for 300D).
Details:
1996-2020 Toyota 4Runner
2008-2019 Toyota Sequoia
OEM PN | 81270-35140 / 81273-35130 / 81270-3512 / 81270-35090 / 81271-95J07

You could cut off the male connector and add spade terminals to the bare wires. Instead, I bought T10 female sockets to make an adapter, to easily replace the lamp in the future.
Amazon: "HUIQIAODS T10 W5W 194 LED Bulbs Female Pigtail Sockets Connector Adapter for Bulb Replacement" $3.99 for qty 4 w/ free shipping. Enough for both my 300D cars.

Awaiting the sockets. Will report back if any install issues, but the screw holes line up. Might need to use screws and nuts instead of just sheet-metal screws, but you can reach the backside thru the inner trunk holes. I wonder why M-B didn't simply use side lamps which fit in the body instead of the separate wiring up thru the trunk lid. A case where the designers won over simplicity?

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans