Check this thread. It had the diagram etc you will need. Basically run a set of wires from the HU front pre out directly to the factory amp. You will probably have to play around with your new amp to get the volume levels to match up between the two sets. The volume level should be much lower than stock depending on how much power is coming out of your new HU preouts. I can turn my alpine to full volume and that is about the edge of where distortion starts coming into play on the fronts.