no room for my subs, 6x9’s as subs?

I recently traded my 2001 Dodge Neon for a 2000 Dodge Dakota.  It is single cabbed, so obviously their is no room for my 12" Kappa perfects or Sub Amp. 

I have a middle of the line (Phoenix Gold) 4 channel amp, 100 watts per channel (claimed) under the seat, powering 2 Alpine type-r 6.5" component sets.  They sound amazing to me, but I want a small amount more bass. 

I have alpine type-r 6x9s left over from my neon.  They will set easily behind the seats and run off the 2 left-over channels from the PG amp.  How can I get the most bass out of the 6x9's.  Any treble is obviously useless becasue it will be killed by the seat.  Can I port the box, or should I just throw them in boxes and call it done?

Thanks for any and all suggestions.....

Jacob

you could cut the 6x9 holes a lil bit, and drop a couple 8" or 7" midbasses in  there. You would need to enclose them, of course, but that would do the trick, they usually don't require anywhere near the airspace of a sub to sound good.

http://www.hifisoundconnection.com/Shop/Control/Product/fp/vpid/1570528/vpcsid/0/SFV/30046

Here's a good set of midbass speakers,

http://www.sonicelectrnix.com/item_11643_MB+Quart+RVF+164.html

and here's another set thats not so expensive.