I have read in multiple places that the factory head unit filters out bass to the rears as volume increases ... supposedly the only right way is to connect at the factory door sub amp.
There is a tutorial on it somewhere ... have to find the link.
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.