I too have the infinity system with the amp. Have blown a few speakers along the way and was told by numerous sources that you cannot utilize the factory wiring to install a aftermarket system. How or why this is possible I'm not sure but my nephew has the same system and he had to run all new wiring when installing an aftermarket unit. I wasn't even able to put in any other speakers other than the factory ones in my system. They just wouldn't work. Has me baffled like you!!!
Rich