Before I explain what I am trying to do, my question is: How do you get to the speakers in the extended part of the cab?

I am thinking about adding a sub to my truck.
I'm not really going for the thump, but I know that I am missing part of my music that the stock speakers just can't play.

What I would like to do is add a sub to the system so that I get the extra frequency range. To run the sub I will need to add an amp.
I found a insert that will put the sub in to the larger of the two storage openings in the extended part of the cab. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5703321536&sspagename=STRK%3AMEWA%3AIT&rd=1
http://i23.ebayimg.com/03/i/01/b3/23/b3_2.JPG

I like this because it doesn't take up any room.
I am going for very little impact.
I would like to put the amp under the passenger seat.

If I am adding an amp, I might as well run the stock speakers through it as well, that way I can limit the frequencies so they are only playing what they should.

From what I have read, my first time doing this, I should run new wires to the speakers from the amp instead of the stock ones, but I need to get to them.

I appreciate any and all comments.