No perfect solution. If I had two fire extinguishers, I'd mount them both in the cab with easy access, like next to the driver's seat, connected to a line that runs to the firewall. A nozzle in the engine compartment and one behind the dash. There are always things to worry about if you want. It's how far you want to go into it. New cars overheat and catch fire too.
In "The great Waldo Pepper, a plane crashes and burns but the pilot is stuck. There was no extinguisher close by.