Wiring Under G. Cherokee Rear Seat

Sounds like you crucified the signal to the amp.  Best bet is to go into that wire harness & repair any damage you did.  And remove the errent screw if you haven't already.

Why put an amp under the driver's seat when you've got that great space under the rear seats anyway?  Where did you pop the speakers from?

OK, better.  Where specifically was the wire bundle, and is it the one that leads in to the amp?  Where did the screw go into it?  You've got to get in there & repair those wires, chances are you've also dug into wires that do other things but won't notice until later.  For instance your taillights or the rear wiper, maybe the fuel guage sensor or rear ABS. 

Also, you mention RCA's - is that what you're feeding the amp with?  If so, they should be intact, no?  then from the amp to the speakers, you're using the factory wiring that's under the rear seat?  Is this where you've popped the speakers from, or from the dash?  Try setting up a new wire to any speaker from the amp & see what happens - don't use the factory stuff.  Did you ever get sound from the speakers before the screw but after starting the install?

Also - what are you powering the amp with - factory wires or your own?  My next guess is that you've cut the remote leading to the amp from the HU.  Try shorting the remote at the amp with a jumper to the B+ amp terminal, that'll tell you.  Also, if using the factory wiring for power etc test those wries too, you may have cut them or blown the corresponding fuse under the dash.