If you mean, behind the seats so the seats face forward - no, as there is ahas anyone ever cut into the panel behind the seats to install speakers,
(6x9) ???
fuel tank, structural steel and more to contend with (including the outside
of the car!)
However, if you mean - as I assume you do - in the back (parcel shelf?) with
the cones facing upwards, then similar things have. 6.5" is the most
common, 8" has been done with some good planning!, but I don't know about
6x9. Are you wanting subwoofers or full range drivers? There are much
better higer end 6" and 8" drivers available than 6x9. One NZer has
installed a single 10" (I think) Sub between the seats in an enclosure - can
be seen in NZ Car Audio mag.
I know Fletcher has a couple of high power 6x9 drivers he thought about
installing on that shelf one day - I think we came to the opinion that it
would be too hard. I have 2 10" high end subs + amplifiers etc that I want
to do something with sometime...
A couple of good links here
Gary Madisons car
http://albums.photopoint.com/j/AlbumInd ... &a=1150565
Lots of sub installs http://users.technocraft.com/~dune/miata/subs.html