Stereo Upgrade Question

[Papa Buddy6 1]

I have a Woofer, Amp and Line converter I plan on installing in the back of my R/T. It has the Base model stereo in in it. I'm reaching out to see if anyone has done something similar and what did you used for the source signal. I've seen up grades using the Alpine system and pulling it from that Sub input that is already there in the back but it is my understanding that connection is not live with the base system.