From my days in the rally car, I strongly suggest being able to reach the extinguisher from your seated position. Only because it's not outside the
realm of possibility that you or others can be trapped in place i.e. crash, rollover, etc. Or that the wiring is already getting fried
and you are unlikely able to lower a power window, hatch or the like at the opposite end of the vehicle.
We always used tanks with metal nozzles (no plastic or floppy hoses) and mounted them low as to not get brained by them in an impact.
I'm a big fan of mounting in front of the passenger's seat bracket, or center console area next to the seatbelt cutter.