The P position on the key switch turns on the parking lights.
Both the key and run/stop switch have to be on to turn on the carb heat. The heater warms the carb throat to improve fuel evaporation and prevent icing. (ask a pilot how important this is) As best I can remember, the heaters shut off when the igniter module reaches around 40F.
The wiring goes from the main fuse to the key switch, then to the run/stop switch and from there to the igniter and carb controller. The carb controller opens the floatbowl vents and, when it is cold enough, turns on the carb heaters whether the engine is running or not.
