I've been shopping for a new GT and it seems all of the have the Shaker 500, is it a simple addition of amp[s] and subwoofer in trunk?

Are all cars wired for the 1000, harness/connectors etc.?

 

I was looking into this very upgrade a few years ago and its way more costly then its worth. You will have to change out the head unit and run wire. Just get a Stealthboxe or a infinity basslink.

 

Yep. Will sound way better, and probably cheaper to boot. IMO, improving sound in these cars consists of (stop when you run out of money/are satisfied):

1) Front speakers
2) Aftermarket subwoofer w/amp
3) Amplifier for main speakers
4) Rear speakers
27) headunit

I did 1-4, and am still running the stock HU. It sounds absolutely amazing.