How did you get there extinguisher on your rear seat belt like that?
Just drill a couple holes through the plastic and bolt it up. I took some thinner metal I had laying around (maybe 10 gauge) and drilled matching holes through it to run the bolts through, so it's bracketed front and rear. In a crash it will not come off unless that entire panel does, which would be near impossible with the seat belt bolt holding it in place. It's a great spot to mount an extinguisher, though not very close to the driver.
GreenTruck...That is the exact fire extinguisher I had. No real way to check them and obviously didn't work. Looks like online Kiddie had a recall on it in 08-09...and they probabably should do another for 09-10 as mine didn't work worth a darn. I would hate for you to need that and you end up trying to pee on the fire to put it out becuase that would have done a better job than mine.
Get an extinguisher with a pressure gauge, and check it at least once a month.
I check mine every time I open my rear hatch (it's mounted on the inside of the rear hatch next to the seatbelt). Sometimes not every month, but usually more often.
As far as other extinguishers goes, you need to be careful. CO2 is often not used because of it's unique properties, namely that it's not lighter than air and displaces oxygen. So say you're putting out a fire and someone has collapsed on the floor inside the vehicle, it'd be theoretically possible that the CO2 could collect inside the vehicle and smother the person without you even realizing it.
CO2 is a good way to put out fires, but even fire fighters tend to prefer ABC extinguishers for that reason.