I decided to put aftermarket subs in my car, and all I hear after installing it *correctly* is a loud humming. I've tried everything (eg. changing amp, changing deck, changing cables and rewiring) and nothing works. I'm all out of ideas, and I need your guys' help. Thanks!

 

There are only one or possibly two things you haven't changed...

Sub woofer and the sub woofer cross-over.