Erik und alles,
    A couple years ago, I wanted to upgrade my 77's stereo. I installed
large(6x9) speakers and an amp without molesting the interior.  I bought a
couple of those felt-covered boxes and dropped the speakers in, installed an
amp under the rear seat sitting right on that converter housing, then ran
wires from the amp to the boxes. This setup has the following pro's:
    1. No hacking for mounting the speakers.
    2. Nice anti-theft quality built in...it's easy to just unhook the boxes
and throw them in the underseat or other cabinetry. Nobody knows they are
there.
    3. It added 2 big ole speakers to the other 4 I had.
    4. wiring is a breeze.
    5. They are foreward of the kids in the rear seat so I don't blast their
little ears just trying to hear them up front.
    6. Those big ole speakers with that amp pushing them will make you
forget about that 10-week , labor-intensive soundproofing job you are
considering. Man can you hear those even on the freeway with the windows
open.
    7. Position the 6x9's anyway you want. Let the wiring be on the long
side and set them outside the sliding door if you like...not recommended
while driving;)
    8. No, they dont look like a cheesy, cut-rate install.
    9. The big blaupunkts make Dr. Laura sound kinda sexy. He He He!

There you have it. EASY, CHEAP, EFFECTIVE. Oh, My other speakers are 2 in
the door panels- just replaced the old, didn't cut there either. 2 more tiny
ones in the very back facing down out of the back of the overhead
cabinet..same story with those.
Jay