Check out this thread, I posted some products that will convert the signal from speaker line to RCA.
https://www.crossfireforum.org/forum...re-volume.html

This is the best way to change all your speakers, install a new amp and keep the stock head unit. The stock amp interacts with a lot of the car's functions from what I've read so it may be best to install a new amp in the hatch or under the seat for your 4 channels in the cabin and you can still have a mono amp for the subs in your trunk.