Trunk mounting is good and practical. I've got one nozzle in the engine compartment and two in the cockpit. Both are pointing down towards the feet so you get initial blast on the firewall, up under dash and then the blowback on the rest of you.
My activation handle is on the dash and within reach when I'm strapped in. I also have a small halon bottle between the seats.
Nothing burns faster than a fiberglass car......
Roscoe