[RWebb]

get a big, clean amp - you would not believe the power needed in a NOISY automotive environment

put sound deadener on at least 1/4 of the surface of every body panel that does nt already have it - esp. the doors where the speakers are

remove the vapor barriers on the doors and use the thermal conforming stuff that Cascade Audio sells - their web site has pics of how to do it and how to use plastic tubing for the cable, wire and rods feeds thru the material - it is a water barrier so don't worry about the old vapor barrier & it seals the doors too

various other panels should get "MLV" on top of the deadener - a heavy layer of vinyl that is floated on top of the deadener by sitting on a layer of closed cell foam

find all open 'tubes' - channels in the unit body such as are formed for the A and B pillars etc. and stuff closed cell foam in them to block sound transmission

track down any other noise sources and deal with them too

this, together with the other posts, will give you as high a quality audio env. as you can achieve without laboratory measurement equipment and skills