How about putting in an ear deafening car alarm?
I finally bought a new CD deck about a year ago after my Kenwood tape deck, after 10 years of continuously cranking heavy metal tunes, died.  I bought a new Kenwood CD player, nothing fancy, the KDC-217S Model which looks exactly like this one here:
When I bought my Sidekick in late 1993, the previous owner too had 6x9 placed in the back above the rear wheel wells.  This spring I am going to replace them with some nice new heavy duty 6x9s along with the 4's in the front which are still the stock speaker that came with it.
I've been maybe thinking of replacing the cardboard and vinyl in the back with some nice carpeted particle board.  I think it would look killer.