Want To Add Sub But Do Not Know Where To Tap Signal? Help

I have a 2011 Prius 3 with the JBL system. The amp is a jbl 86280-0w622 p5. I am trying to wire in an amplified Bazooka BTA10100 10" 100 watt tube. I am stumped on where to get the signal from? I have looked at multiple threads about this issue and nothing has worked for me. I am getting a high level signal but nothing seems to work with the tube? I have even tried a low level converter to the high level signal and it will not work? My phone connected to the RCA's works perfect so there is nothing wrong with the tube. Should I tap into the front speakers at the speaker itself? Again, I am stumped and just want this to work. Thanks