If you are willing to do a box, the possibilities are endless.....

Figure out which sub you would like to use and we can give you some options. You can always mount the amp under the rear deck between the 6x9's, up out of sight. If you get a a decent 4 channel, you could bridge 2 channels for the sub and use the other 2 channels for your front and rear. You would lose your L/R fade from the headunit, but it can be setup properly.

We will need to know what sub you want to go with to spec a box, mounting depth and cuft reqiurements are most important.