I don't think you'll be wanting a functional seat when you have subs and amps and whatnot back there.. Passengers tend to be very unappreciative of the hard work and money you put into things and mostly aren't careful enough..
That being said, I always thought of the jump seat as a perfect place to install one sub (and I don't think those trucks really need more than one, any more is overkill).. A well planned out fiberglass/mdf box there would be sweet.. should blend in and stay out of the way while giving a nice kick..
Check out
www.s10forum.com and see some of the stuff guys there have built, also.
Good luck.