I don’t believe anyone knows where a fire will start. True, a fuel fire will only be in the engine bay but an electrical fire could be anywhere in the car with the right, or should I say wrong, situation.
I chose to mount them in the cabin at the far outer edges of the firewall because:
FIRST - I know I’m not going to fight a fire while sitting in the cabin - I am going to get out of the car to fight the fire where ever it is.
SECOND - I want to be able to get my hands on that extinguisher as quickly and as easily as possible. Mounted in the spots I have chosen, the unlatching and retrieving of the extinguishers is easily accomplished as they are at a working level and exactly where you are when you exit the cabin.
I have seen some extinguishers mounted on the floor directly in front of the seats. I don’t like this location because it is difficult to unlatch and retrieve from that position due to two legs being in the way. You could always exit the cabin and then bend down and re-enter the cabin to unlatch an extinguisher and take it off the floor (however long that takes), but if the fire is in the cabin are you really going to want to go back into it?
If the extinguisher is in the front trunk that means pulling the release, opening the hood, un-latching the extinguisher ( is it mounted in the open or are there towels or a car cover or something on top of the extinguisher ?) and ...now... you can begin to fight the fire.
If it is in the engine bay are you really going to want to open the decklid ( only to perhaps find that it is locked and the keys are still sitting in the steering column?) and then reach into the burning engine bay, unlatch and grab the extinguisher?
My 2¢
Larry