Depending on how tall (or short) you are...  One of these may fit behind
your driver's seat:

http://www.crutchfield.com/S-mZ8EpY7Os25/cgi-bin/ProdView.asp?s=0&c=10&g=178650&I=044QLTCL12&o=p&a=0&cc=01&avf=N&search=

That way its easy to remove if you need the space otherwise.  I use one
and its pretty convenient and sounds awesome given its narrow size.  I'm
about 5' 5" and it just fits.  I think if I were any taller it wouldnt
be good, I always have to remove it before taking it in to a shop...