Not alot of science needed, just drive the car and listen, the STI is a very loud car and it takes a bit more effort to track down the source of the noise. Since I have already done two deadening jobs on them I pretty much know where the issues are, why I wrote a specific guild for them. In general though:
1)All cars have alot of road noise from the doors, the best way to combat it is sealing up the access holes, mating and ensoliting that area. None needed on the outside if you do not care about audio or have no speakers in that particular door.
2)Sedans get a fair amount of road noise from under the rear parcel shelf, if no subs in the trunk then just seal that up really well, the STI is terribly noisy in that area.
3)Wagons and hatches are a bit tougher to deal with but treating the hatch like the doors is all that is needed for them, then the rear sides on the more resonate prone sections. I like to then do as a minimum the floor but you can cover just the less supported and more resonate areas if you are on a strict weight control plan for the car. Then cover all areas with ensolite. You can put some reall accountical egg crate style panels in voids as well.
4)Lower A pillars next, seal them up will
5)Flor, minimally would be over the resonate areas, softer, give them a thump and see how they sound or just cover the whole floor, it will be even better. Then ensolite it.
6) Roof, on most cars not much of an issue, on some very loud in the rain as has been attested to here, just lining the roof and then the headliner with ensolite will help to a great deal if you do not want to add mat there as well, best to use both but at least just foam will help, I have done so on my projects. Another trick is to bond aluminum channels to the roof then cover with foam, my last one was actually done that way.
Our current project has a one piece carbon/kevlar roof that is already quiter than a steel roof, weight is a major concern for us so we put on one layer of ensolite, one layer of aluminum foil then a second layer of ensolite, worked very well and less than a pound of weight added.
Once you deaden one area well and drive the car the other areas start to stand out more, do the next, hear the remaining ones more again. Once all the primary areas are done then you can focus on any smaller areas, they will be even easier to identify but sometimes difficult to fix.
Then, I like to use a test CD and play individual requencies and low, moderate and high output levels to excite and isolate parts that are prone to resonate while driving. Very easy to find them this way, then use whatever method needed to deal with them, silicone, tie wraps, ensolite(or other foam) double faced tape, etc. Once those are done you are pretty much on the road to a nice quite ride
I have to get back to work, finish packing our glass roof and hood and shipping them out then back to fiberglassing sub enclosures