Aftermarket speaker/head unit install questions.I am going to be intsalling a

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I am going to be installing an aftermarket head unit soon. I believe that I would like to run new speaker wires to the door panels and sail panels. When doing so, where do I splice into the factory harness? Do I just crimp the speaker wires directly to the harness on the head unit? Is it possible to just crimp female speaker terminals onto the wires going to the existing door speakers so that I don't have to worry about running new wires into the doors? My car's factory bose system does not have speakers in the sail panel right now, so is it even possible to splice into the harness somewhere and install speakers in the door panels or could I just crimp those speaker wires straight to the harness that will come with the head unit too? Any and all help/input/advice is GREATLY appreciated! I really don't understand car audio at all lol.