The workers need to be able to reach it so buried deep inside the car isn't great. From lots of experience within the drivers reach is a good idea if possible. Otherwise you will constantly have to climb back out of the car when you forget to turn the switch on. It's also nice to be able to turn it off after a crash even before you unbuckle.
I'll also say having it on the exterior of the car isn't the best spot. I had one there and in a crash it got hit. That could make it impossible to turn off. In my case it did turn off from the impact and the first worker to arrive turned it back on thinking they were turning it off. Watching the lights in the dash come back on while I was upside-down with gas pouring out of the carb right in front of me is a experience I won't ever forget.
So best bet is somewhere reachable by the driver and workers that is also protected from impact.