help installing amp in 07 camry

Hi, i have an 07 XLE camry with a navigation as the headunit, I want to install a JL audio sound system, a W7 sub with a jl audio 1000/1 amp. I dont want to take out the whole navi from the front to install the RCA or remote wire...someone told me that I can take the rear speaker wires and do something to attach that into the amp so I dont have to take the front off. Do any of you know how exactly this procedure works, if theres a big difference in doing that, and if its worth it. Also i'm assuming Toyota already has a amp in the car because it came with the JBL sound system, by installing the amp that way will it interfere with the amp already in the car.

Check here:

http://www.installdr.com/

that site doesn't have the year for my car