If you are keeping the stockhead unit you have two choices.
You can use speaker level input to your new amp. This means taping into the front left and right speaker outputs and running both to your new amp.
The second if using a LOC such as from PAC or JL "not the clean sweep" This you also will tap into the speaker outputs but than run a set of RCA's to your new amp.
I prefer method 1 just make sure your new amp has speaker level inputs.
I actually did this in California with a kenwood sub and a boston acoustics amp.
California build thread