Welcome to the forum! It's not under the passenger seat, but under the passenger side carpet closest to the firewall. I'm not sure if the base models had an amp or not, but if you have one, that's where it'll be, you'll pull some carpet back, then some styrofoam, then you'll see the amp. But with the speakers...since you CAN hear them, even though very quiet, i'd doubt they're blown, as you'd hear either nothing at all, or it'd rattle like hell. I'm going to guess, along with the static, that at some point your speaker wire was getting pinched, possibly in a door, under the floor, in the door jamb possibly...and they've gotten to be exposed metal, which could cause a ground loop.

But a busted amp does sound like a reasonable cause, try hooking the speakers up directly to your head unit and see if they work. The reason this may have happened, if it did. Is that you're most likely running a high level input through an amplifier that's designed for low-level. Gives you some extra volume, yes, but also creates crazy amounts of "clipping," which kills amps. If you look and see that you DO have an amplifier, let me know, as I have one I could sell you for cheap, they're hardly ever on ebay these days, and when they are they're like 400 dollars IIRC.