Hooking amp/subs to factory radio on CTS

I recently bought a CTS and I love the radio that's in it ( factory ). I have made up my mind that I am keeping that, but I want to hook up an amp and subs in it. My question, where do you run the amp wire from the battery? In my Lexus - we just drilled a hole in the firewall, but I'm not to keen on doing this to my new car, so is there anywhere else we can run it? Also any other suggestions on hooking it up? Thanks so much!