Does anyone know what this switch is for?
James and Suzi,
I can't tell you specifically what that switch does, but if you do a search on the Forum for "secret switches" or similar terms you'll probably find your answer. For instance:
secret? switches inside dash (https://www.foreforums.com/index.php?topic=37565.0)
Three switches under dash cover? (https://www.foreforums.com/index.php?topic=33003.0)
Mine alters the idle speed. Edit to add: FT had so many variables. Try looking at the wiring schematic to trace where it goes or is named.
A single switch inside the dash, especially if close to the tank monitor/rear camera area is potentially the switch to ground the tank monitor sensor readouts when you calibrate them. In the link below, it's a convenience switch to momentarily ground pin 15.
https://www.foreforums.com/MGalleryItem.php?id=3455
If you still have the old Javalina pressure transducer tank sensors, don't flip that switch until you trace the wires to it or you might muck up your tank calibration and you'll have to recalibrate them (oh what fun..... :-X )