Line level input to JBL amp

The short answer is "no" unfortunately.
The signal coming from the factory head unit to the JBL amp under the passenger seat is a digital signal. To get a suitable signal for an aftermarket amplifier you're going to need to work with the electronically crossed-over, equalized, and DSP'ed (time delayed) high level signal coming out of the JBL amp. There are a few ways to do this: some have used a device like the JL Audio SSi and CleanSweep...you could also use a processor like the Audison Bit One, the Rockford Fosgate ThreeSixty.2, Audio Control DQL-8....but probably the best tool for the job would be the JBL MS-8.

The other option is to scrap the factory amp & head unit altogether and go with an aftermarket head unit like many folks on this forum have.

 

I'm not sure about the format of the digital signal. I've read about devices that let you jump into the signal path in a couple select German-made vehicles (BMW & Mercedes), but I haven't heard of a similar animal for Toyota.
The hi-level inputs on your amp may be rather tricky, since none of the signals coming off the factory amp are full range and un-processed. I suspect that the 6x9's in the front doors are the closest to a true sub signal....ideally, you'd be feeding full range signal to the aftermarket amp.

 

well this sucks I was hoping to keep the factory head and using a full signal to my amp/ sub.
right now I'm using the outputs of the jbl amp but its sending an amplified signal causing my loc to clip very early

 

nvm scratch that the amp outputs are fine to use but they do have the processing from the amp which may or may not be desirable. I haven't noticed anything unusual about the rear channel in terms of freq response for my subwoofer