My 200tdi has a isolator switch mounted in the passenger footwell.
A few weeks ago one of my mates knocked it, and turned it off whilst i was driving and i didnt even notice until i came to start it again.
Fortunately this was during the daytime, obviously no lights etc. If it happened at night it could have been a different story..
I would advise you to mount it well out of the way, where it cannot be knocked or played with!