Mine is mounted under the floor on a bracket I made from 1/8" aluminum, screwed to the bottom of the wood cross piece. I then made an extension to the shaft where the handle mounts so that the handle is just above the level of the carpet under the front edge of the seat. Yes, a hole drilled through the wood floorboard behind the steel cross member. Very unobtrusive and easily accessed.
With the switch there, I can easily disconnect the entire system from the battery (which I do as I exit the car).
Problems of concern that the switch being easily accessible address:
* A stuck starter switch ( have seen happen).
* Stuck brake light switch.
* Reverse drain through alternator when not running.
* Short behind the dash panel.
* Theft prevention.
* Forgotten headlights or turn signals.
(I'm sure there are others, I invite contributions.

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Edit to add a picture showing the switch through floor board, no carpets installed at that time.[/QUOTE
With regards to the reverse drain to the Alternator, explanation please? I have one of these switches and also have an alternator...does this mean I need to shut off the switch every time I drive the car and stop somewhere like a car show? I had to get a jump start today and wondered if this could have happened