Just where you have described. My reasoning was that I can grab it on the way out & then decide if I want to come back in! There was also the idea of having one in the cab too, which I quite like the idea of in case of an engine or cab electrical fire. I may well buy anther one to place on the hanging locker side behind the passenger seat, or possibly on the floor behind the driver's seat.
Word of warning; it isn't always a good idea to put the fire out . . . . just sometimes, it could be advantageous to let the damn thing burn & then claim on the
insurance.

But you do need to make the decision quickly & if your cover isn't up to it, then the wrong decision could be disastrous.
Many years ago I actually used an extinguisher on a car fire. The car had stopped at a side junction with smoke coming out from under the bonnet, I stopped, grabbed my extinguisher, ran to it & paused briefly before tackling it - are you sure you want me to do this? I asked. Yes, Yes, YEEES, was the response, so I popped the bonnet & squirted the powder over the seat of the fire.
I put the fire out, but as described by Runnach, it was a mess & the car was not driveable anyway. I reckon he might have been better letting it burn & claiming for a total loss!