this right here.
AmpPRO 4 Amplifier Integration Interface
catalog.pac-audio.com
no cutting of wires so it's really easy to return to stock. retains all factory functions and gives you more control over some things (parking sensor volume, small EQ), most importantly it will sum and flatten the signal to your amps. Lots of options with this. You can do just an amp/sub with the rest running off factory. Or upgrade front/sub and run the rears off factory. Or upgrade everything. Just be sure to disconnect the speakers you no longer want sound out of.
I use this with a dsp (toslink adapter) with 2 amps, 3 way up front, rear fill and a 12" sub. Blows the crappy little alpine system out of the water.