You can use the low level outputs on the factory HU as the inputs for your amp. You'll have to cut an RCA cable to do it. Then, wire in the Pioneer amp's outputs AFTER the factory amp (bypassing it) or run new wires directly to the new speakers.
Figuring out the low-level leads from the HU and connecting them to an RCA cable might be a challenge unless someone on the board has got the color codes (you might want to use the search feature).
