[AftermathAlero]

you need to tap in the wires before it hits the monsoon amp. Like behind the headunit or on the monsoon amp harness get the wires coming in from the deck. Since your taping from the rear speakers its has the amp output and not the signal line you need.

I just did the same for this infinity g35 i'm working on w/ the bose 8 speaker system & bose amp. I cut all the wires on the bose amp harness and tapped into the stinger line out converter from the radio, then amp output wires to the speakers, i connected the amp speaker output wires to that so i didn't have rerun new wires and used the old factory 18 gauge wiring.