The relay is located behind the instrument cluster on the SD. Once you remove the cluster, it may be behind a bundle of lines, and kind of tricky to get to.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.