I'll second mike in dp.
The way to do it is keep you switch, but wire in a relay so that the relay switch and manual switch are wired in series. I.e. both have to be closed for power to reach the fogs. Then pull signal for the solenoid from a switched wire or fuse socket.
This way they obey your interior switch, but shut off with the ignition.
Make sure you get a relay rated for enough amps to run the lights.
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