You can do this with the stock head unit. There are two ways, depending on the amp. Some amplifiers have speaker level inputs. With this, you tap the speaker wires and it feeds signal to the amp. If the amp you're getting doesn't have this, you can use a line out converter (LOC) to do the same thing as the speaker level inputs. I don't know that it'll help, but check out the link in my sig for my sub/amp install. I used the speaker level inputs on the amp.