For $500? Good luck with that.
If you wanna get subs into the trunk, you need to buy a head unit with RCA outputs, an amp, subs, sub box, and all the wiring. I don't know how you can do that for $500 unless you put in all crap like Pyramid. Plus installation if you have someone else do that.
Expect to pay $1000 for a decent setup.
-Diabolik