OEM subwoofer

I haven't seen that done. I would think you won't get far without a amp and a decent enclosure though.

I don't have one and haven't seen it, but I do a lot with stereos, you would be looking at a few issues. Most likely the ohms wouldn't be correct for what you typically find in the aftermarket. You can buy 8 or 4 typically, it might be right though, honestly don't know without a schematic. They there's the fact that I haven't seen anybody selling an 8" subwoofer in a long time. The last major one I see is that the amp won't likely push anything bigger than 100 amps or something, again no idea how big it would be, but not very big. I'm willing to bet that you would be better off running an aftermarket amp setup, etc. Sorry, trust me, I know how the tight money thing goes...

Try filling the OEM sub enclosure with polyfill.