The potentiometers could be for either the heater or for the defroster but in most models that had under dash controls the heater was the switch illuminated. From the looks of the switch placement they were probably installed by a dealer and where a control was located could have been a driver decision.
Packard says the 3 position switch above the manual choke is for the instrument or panel lights and reading light. It is fed from the headlight switch so that needs to be on for those lights to work. No idea on the dome light as before the 22nd series Packard usually did not provide much if any body wiring information as part of the documentation that went to the field. Body wiring is shown on a miscellaneous drawing that is body specific and was not part of ordinary info. Kev has some photos and info on the dome/reading light switch in his 37 project blog.
Could the ordinary toggle switch be for the dome light or do you have an electric fuel pump it might control?
Here is a very nice 37 drawing of the 120C that according to the file info notation was put together by Bob Johnston. I didn't see this particular drawing in the wiring diagram archive and not sure how it might be labeled so it could be somewhere else.
Attach file:
120C 1937 Wiring-drawing.PDF Size: 298.66 KB; Hits: 39
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