I don't know about the seat, but the rear dome light in my MB is operated by a switch on the front console, next to the antennae switch. (It's not a 420SEL and has no rear doors but yours may be similar) The doors don't operate the rear light. Not sure if they do on the sedans or not.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick