there is a two prong spade connector in the back of the ashtray. Remove the inset of the tray, and beneath it you'll find two phillips head screws. Remove those and slide the whole tray out. Tighten up the connector, reinsert the light, and reverse the procedure.
sliding the ashtray out will pull off the connector if it isn't tight enough.

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'79 300D, Ivory "Gunther" ......going
'81 300SD, Metallic Blue Silver, The Grey Ghost.....going
'87 300D Turbo ...gone
'05 CDI.... gone