There is no dedicated line-level out for the subwoofer. The factory amplifier integrates the low-freq from the audio unit with the ANC (noise cancellation) signal to make the output to the factory sub in the rear. More than a few who have tried to integrate an aftermarket sub by tapping the left and right (low but not really) line-level signals have had to disable the ANC system completely to prevent some pretty impressive but definitely unintended low-frequency effects; the ANC system treats the aftermarket sub as a noise source and tries to "cancel" it.
Vist the audio forum here for some better hints and maybe some specific guidance on what works. You aren't the first owner looking at improving the sound with an aftermarket sub.
I am not so sure about this. On models that don't have a separate amp module, this is definitely the case - ANC integrates into main audio unit which houses the sub amp. One has to take out the connections from ANC microphones in front and rear to handle this. (Connector E, pos 5/6 on regular audio unit)
But on Touring - or any model with 'premium' audio, which has a separate amp module - Sub output is actually normal pre-amp. On audio-nav unit, looking from back, it is second 24P connector from left along bottom (D) with two rightmost pins (12 and 24, red and lt blue) being Sub Pre + and - - and those should be just your audio lows, and can be sent to the new sub amp. ANC is handled by a separate module that takes in signals from the microphones and forwards them direct to the amp module. If you are taking out OEM sub so that amp module has no sub to send to, you should be safe from "impressive low-frequency effects". But to be on the safe side, you can disconnect the ANC module, which is in the bottom of the center console on passenger side, about level with seat front mount bolt.