Hi, I'm Ted, one of the safety crew. Ideally, the extinguisher should be located near you (as was already mentioned) so that you can grab it quickly as you're exiting the vehicle. If it's somewhere else, like in the back, it may not be accessible should there be a fire in that area. Also, it should be easy to get out of its holding device, usually with a snap latch. Last, it's good to have the fill conditon indicator readily visible to make sure it's usable when needed.
I've had to use one several times, sometimes for others and once for me when I had a gas line break on a former car. Very glad to have it. Remember, too, that in a pinch, or if the extinguisher runs out, you can use sodas (preferably carbonated), beer, or champagne (doesn't everyone carry some?). Shake vigorously, open and point. You'll see that method used on winning race drivers, probably so they won't combust.
