Some premium audio systems from factory do oddball things. Like have the crossover for the tweeters embedded into the side panel speakers. Or could be other way around. The factory subwoofer in my old cobalt got its signal by a mono channel split off from the rear speakers or something. My new car is wired differently. But, its nothing that can't be overcome. Best you consult someone at a shop who is familiar with your systems wiring. They can tell you if it's a straightforward job or something more involved