Hmm, I've never actually measured it but I'd be surprised if you cant' get the 12" in your car, what do you drive? I got mine into my friend's volkswagon Golf,
http://www.vw.com/golf/. The mirror housing sits on the back seat, restrained by the seat belt, the secondary housing sits in the dob base, which in turn fits in the boot.
If it hasn't been posted before I get home from work, I'll measure it up tonight.