[Jazzzed]

I would test those speakers with something other than the car stereo as well just to confirm that they're not blown. They look like they're wired in even if it is a bit of a hack job. Then on the amp's wiring harness, it would be tough to screw that up since all you would have to do is match the color coded wires together in order to reinstall the old amp. If there aren't wires dangling at the amp, then you should hear SOMETHING out of those speakers, even if it does sound bad. Do you even hear any hiss (or noise floor) in those speakers? I'd start there and verify that you do have two working speakers first. Beyond that, you are going to need a new amp to run those Fostgates or any other full range speaker for that matter, so a new amp is a given.

And that amp runs the front speakers too, right? So it is getting power, and properly wired to the front speakers. Are you sure that you don't have the front/rear fade setting in your radio turned all the way to the fronts?