There's a good chance your car is still okay. When you reverse the polarity on an electronics device all it does is reverse the direction of current flow. There's a chance your AMP has an ESD diode which might have been biased on in this case shorting your positive (12V?) to ground (negative) and the fuse will be zapped.
Have you tried one of those tools that scans the car for fault code? If you can still read something then I think your car is still okay. Just need to hunt for that dead fuse.