Here are some old threads on the topic:
Additional Vehicle BOB

Keeping a fire extinguisher in you car

What’s in your car?

Dry chemical fire extinguisher

trunk junk

That's good enough. There were lots more.
[rant on]
Side note: I do not mind experience ETSer's bring up old topics, and I don't mind the new guy's who ask an old question. What I do mind is when newbie’s don't take the advice of the oldbie's. It took about 5sec to find all this info.
All they have to do is click on search type in a few key words i.e. "extinguisher " and boom tons of info. [rant off]

With that said welcome all you newbie’s and I hope you stick around and Use the search feature!!!