Do a search on "Basslink" - Including your post there are at least 19 hits. The basslink is designed to be corner loaded to get the best response - pull up the manual on infinity's web site. The interior of the AV sort of prohibits optimum placement.
Kodiakz has the best solution - mount the subs in the midgate with the enclosure in the bed. Not the cheapest solution - but for true bass - the best solution.
Also, for all of us with the Bose system, the line levels off the factory amp are not full frequency signals. There is a crossover in the amp near the "bass" speaker under the center console. The best solution I have found is to crank the bass all the way to the right. Bose designed the system to play with minimal distortion throughout the entire adjustment range. Bass one bar short of the maximum level and treble one left of center. Seems to be the best adjustment for the 03 AV. My 2 cents.
I will probably rip the system out and start from scratch with a audio/video system as soon as the wife gets a real job and frees up some of my $.