Get a cheap set of Creative Labs or Altec Lansing 2.1 speakers (that include a 5" sub) and hack the satelite speakers apart. You can mount those in your cab, or, if you want some real sound, pick up a set of car audio speakers with the same impedance (usually 4 or 8 ohms) and just do a swap.
There are several tutorials floating around the web, and a few minutes on Google should put you in the right direction.