this would be a custom intall for sure. I believe the amp is digital to the hu.
I found the JBL amp on the right rear corner on the floor pan. Yu have to remove the black tray that holds the jack/ parts. It is right there in the corner. if yu can tap into the JBL harness without problems, then the right rear panel would be a nice close spot for the Stealth Sub. But I don't believe the amp is where yu can get your line out??? That is the key to this project!!! Not sure at this point. Trial and error with a Volt meter and schematics of the JBL harness might yield the line out.both channels.
getting to the rear door speaker outputs might be a solution!
The only problem I can see, is finding a low level line out for the amp.
you might have to buy a
high level TO low level converter to drive the sub amp. And just tap into the left and right channels with it.
LT

Good luck