Okay, I went beast mode and solved this.

Pulled the cluster once more. I’ve long suspected whatever alloy the temp gauge shell is rolled from is unfriendly to solder. I wrapped a thick copper wire (speaker wire) around the shell and twisted it tight till I couldn’t move it with pliers. Soldered a wire from this ring to the spot on the board. It’s hideous. Shameful in fact.

That said, it’s 82 degrees and cool.

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1981 240D 4sp manual. Ivory White.