I had the same problem recently.  Went to my local Wal-Mart. Bought a
roll of carpeting in their Home decorating area.  It was 10 x 12 or
something like that.  I then pulled out the middle seat, lay the new
carpet over the old carpet and with my exacto knife cut along the
pattern of the old carpet (of course I pulled the trim that was affected
by the carpeting prior to this BTW).  Then I yanked out the old carpet,
put the seat back in, then the new carpet and trim.  Cost me about $25
for the carpeting and the exacto knife.  Two hours approximately.  Good
luck.