You are correct sir. Get the signal that goes into the door sub amp. If you tap into the rear speakers then the low frequencies WILL be filtered out, and this is on top of a decrease in bass as the volume rises. Some people may not notice the loss of fidelity from the rear signal, but it is pretty obvious when listening to bass heavy tracks.
Then I read this: 2012 Shaker 500: how do I add powered subwoofer? - MustangForums.com
This guy said he used some signal analyzer and the full range is definitely to the rears! What a PITA!