KAC-7205) that allows speaker level inputs which I spliced into the rear door speaker and it has signal sensing so you don't even have to run a on/off signal making the install super easy. Just run positive to the battery and ground to something in the trunk. Install took me about an hour.
With the added sub you can turn the headunit bass setting way down to get rid of the muddiness from the mid range and liven up the whole system nicely. Speakers tend to work most efficiently when they only have to work in the small frequency range for which they were intended.