Looking for a replacement grill and low output on the sub.

 

Ok, if the sub doesn't seem like it's working anymore make sure it's plugged in securely to the harness found in the spare area.

If you're adding an aftermarket sub you will need to run a power wire, ground, etc for the amp that will power the sub.
I'm not quite sure what 'fuse' you're looking for.
Adding a sub is really easy especially if you have a hatch. Read up on the threads here and other forums that will basically walk you through the process.