I just purchased a 2012 Silverado last month. I'm looking to add a sub (or two) to the stock stereo system... but I don't know if it's possible? I don't want to get rid of the stock system because it seems like so much goes through it and it would be more hassle than it's worth.. OnStar, USB, reconstructing the dash, and making it a target for thieves because there's a new stereo in the dash.. So what would be needed to add a sub-woofer to the factory head unit / system? Where can they plug into?