Well I added my subs to my SS and they sound fricking awesome. That much bass added to the Bose system makes it all sound really nice. I actually used the box and speakers from two cars ago. It was a box I custom made, and it fits absolutely perfect in the SS. Like it was made for it. I mounted the amp on top of the box. I'd guess the entire box with two 12" subs and amp probably weighs around 80-100lbs. So yah it adds a little bit of weight to the car but thats ok.
I'm including photos as I went along. What I did was I completely disconnected the two rear deck subs and tapped into those two speaker lines and ran them into a line out converter (LOC). Then I ran the RCA cables from that straight into my AMP. Since those two rear deck speakers are subs, that means the Bose system is already taking care of the low-pass frequency filtering. So no need to filter anymore, just run them straight into the amp.
After that I ran a negative and positive straight off the battery to feed the amp. The battery connectors on the SS are kind of cool, to where they have accessory posts built into them. I had to use a nut on one of the posts since it didn't have one.
For a remote turn-on 12V+ I used the fuse in the rear trunk fuse panel labeled " RAP ACCY" Maybe that stands for rap accessory? Because thats exactly what a subwoofer usually is! Anyways, go to Autozone or Advance Auto and buy a "ATM Add A Circuit" which is located in the fuse isle. Remove the 10A fuse out of that port and plug in the add a circuit. Replace the first fuse with the 10A fuse that you removed from the stock fuse panel, then install a 3A fuse for your remote turn on.
The orange plugs are two extension cord ends that I got at the hardware store. I wired the 12V + and - and turn on leads in there. That way if I want to take the box out I can just disconnect that, and the RCAs and it comes right out.
Thats about it good luck.