That depends on what head unit you are using. If you have an aftermarket unit, it should have an signal lead for your amp. That's your best option. I'm not sure if the factory unit has anything that would work. You would have to check the wiring diagram. These can be found at auto sound stores or on the packaging of after market wiring harnesses.
The option suggested in the previous post would work too. There are also switched power items that you can tap into in the fuse box.
Integrating with these newer built in stereos, kinda sucks. Good luck.