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The old TRH housing was a bit loose. I'll take it apart now to check. Gordon noted that the fan had been running when he removed it.
P.S. It appears that the fan was wired backwards. When tested, air was blowing down from the plate. Also noticed that the black/red wires from the fan were opposite those from another spare in the trailer. Have marked this sensor to be fixed in Boulder (since it isn't obvious to me how to cleanly rewire the small black connector on the end of these wires).