i think i know exactly what you should need.
i will use links from sonic electronix, but most of this stuff is available from other places as well(and sonic is not a authorized dealer of all the brands they sell...)
you mentioned both a line driver, and a volume control, so, first, i must mention the JL audio cl-rlc. they are kind of pricey at about $50 each, but allow the volume knob to be remotely mounted from the main controller box-- a huge advantage over other units if your 'puter and amps are in the trunk.
and my review of it: http://www.mp3car.com/vbulletin/revi...ed-preamp.html
and now my mouse messed up and it posted...
anyways, there are also line drivers like this sound gate one, but the differnce with this is that the audio cable must be routed through it, so if your puter is under the seat, you would first need to route all of your audio cable to wherever you want the preamp(this a 4 channel version, the other 2 that i listed are 2 channel)...
i know i have also seen some volume control boxes that are very similar to the soundgate module, just wouth out the preamp part...ah, found one:
then, there are also preamp eq's that you might also be able to use for this..
it all depends on your setup, but i still highly recommend the cl-rlc for its remote mountable capability.
those are the primary options.. there are always a couple that i miss, so if you find something else, do think that it is a poor idea...