I would have to mess with the unit to figure things out but it sound like a amped signal input into the OEM amp which in turn blast speakers. It should not be that hard. I mean a amp just need a signal and they it increases it and pushes it to the speakers. According to the metra harness diagram, you just need to hook up the front speakers and the sub and it should at least play decent.
Who Wants Bose Wiring Diagrams!!
IDK, I would personally have to mess with it to figure it out.
YzGyz