Once you get into your door speakers, you start getting into a lot of money and hassle. The cheapest and easiest solution in the Mopar Kicker upgrade. This is the only drop-in solution. There aren't full-range signals in your doors (far from it). The front doors have low-mid range only. The rears have mid to high with treble boosted. Thus, you can't simply drop in a coax. You'll need a summing device and aftermarket amp in order to make your door speaker replacement worthwhile.
After your dash speakers, your next easiest upgrade is a real sub and amp. You don't need a full-range signal for that. Simply tap into the existing sub wires for input into an LOC.