I've had to use fire extinguishers on several occasions, though fortunately never on my own vehicle. An oily or gasoline-driven engine fire can overwhelm a 2.5Lb fire extinguisher, which is one of the reasons why I now carry two extinguishers in the Jeep. The best location is within easy reach of the driver's seat. On some vehicles (such as my Jeep) you have room to attach a bracket to the outside of the base of the driver's seat. In other cases you can either mount the bracket through the floor or, using a mount like shown to the right, to the seat bolts in front of the seat. I've also seen home-made versions of this made of angle stock and flat plate, either welded, riveted or bolted together.

I have one extinguisher mounted on the side of my driver's seat and the other on the tailgate.

A plastic bracket in a vehicle is a disaster waiting to happen, and I can't understand how manufacturer's can be allowed to sell fire extinguishers labeled for vehicle use that rely on a plastic mount. Metal brackets are available separately in a variety of configurations. Search with Google Shopping for "Metal Fire Extinguisher Bracket" or "Quick Release Fire Extinguisher Bracket" and you'll find options from $6 to over $200.

I've never used Halon/Halotron on a vehicle. I have seen it used in a computer room. The benefit of Halon/Halotron is no residue, but I've been lead to believe it is not as effective outdoors as it is in an enclosed space. It's also expensive, around $200+ for a 2.5Lb canister.