Adding New Amp an Sub to factory equipment

OK so I'm new here so sorry about repeating a question but I'm in a bind an don't want to move forward without a little more information for fear of messing something up. So I just got a 2012 Jeep Patriot Latitude it has the Boston Premium Audio system in it the radio quit working so I put in a new one. I had to get a Crux CS-CR41 Wiring Interface for the radio it's ok but I'm looking for more bass I have 2 12" subs an amp an was wondering how much harder would it be to hook it up along with the boston Audio system? Would I just run it like normal hook up the power line from the battery through the firewall under the carpet to the back rca from radio with ant wire to amp with ground to seat bolt. Or is there a way to run the power and ant splice an or wire from the factory amp to the new amp an subs. Or worse do I need another 2 to $300 dollar harness any help would be greatly appreciated!!!

 

If your stereo has a low level RCA pre-amp outputs on the back, you should be able to hook it to your sub amp. Amp power from battery (fuse it at the battery connection) and blue power antenna wire at stereo to turn on the amp.
Without the RCA pre-amp jack(s) on the stereo you'd need a line out converter https://www.audiocontrol.com/car-audio/factory-system-upgrade/lc2i/ that can handle the hi-level speaker lines out and convert them to low level RCA.