No mention of engine or trans?
The switch is an accessory for the courtesy lights. The interior lights, under the dash, can be turned on/off with the door closed.
That little red light to the left of that light switch is a parking brake light. Good to have as it is lit when the parking brake is on. My last 63 had both of those options.
I do believe that this was only an option for 1963’s. The door switches for the dome lite were only put in 3100 series cars also. The 3000 series cars turned on the dome light with the rheostat on the headlight switch only. The 1962 and 1963 convertibles also had rear seat courtesy lights. My memory is getting fuzzy, someone might have different answers.
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